Check zpool status, see which disk has failed:

# zpool status

  pool: zroot
 state: DEGRADED
status: One or more devices has been removed by the administrator.
        Sufficient replicas exist for the pool to continue functioning in a
        degraded state.
action: Online the device using 'zpool online' or replace the device with
        'zpool replace'.
  scan: none requested
config:

        NAME                      STATE     READ WRITE CKSUM
        zroot                     DEGRADED     0     0     0
          raidz2-0                DEGRADED     0     0     0
            da0p4                 ONLINE       0     0     0
            da1p4                 ONLINE       0     0     0
            da2p4                 ONLINE       0     0     0
            da3p4                 ONLINE       0     0     0
            12385660660938982245  REMOVED      0     0     0  was /dev/da4p4
            da5p4                 ONLINE       0     0     0

errors: No known data errors

After physically replacing the disk, verify dmesg output:

# dmesg | grep ^da4

da4 at ciss0 bus 0 scbus0 target 4 lun 0
da4: <HP RAID 0 OK> s/n PDNLH0BRHA0725   detached
da4 at ciss0 bus 0 scbus0 target 4 lun 0
da4: <HP RAID 0 OK> Fixed Direct Access SPC-3 SCSI device
da4: Serial Number PDNLH0BRHA0725  
da4: 135.168MB/s transfers
da4: Command Queueing enabled
da4: 953837MB (1953459632 512 byte sectors)
da4 at ciss0 bus 0 scbus0 target 4 lun 0
da4: <HP RAID 0 OK> s/n PDNLH0BRHA0725   detached
da4 at ciss0 bus 0 scbus0 target 4 lun 0
da4: <HP RAID 0 OK> Fixed Direct Access SPC-3 SCSI device
da4: Serial Number PDNLH0BRHA0725  
da4: 135.168MB/s transfers
da4: Command Queueing enabled
da4: 953837MB (1953459632 512 byte sectors)

Take the disk offline from zpool:

# zpool offline zroot 12385660660938982245

It should now show the disk is offline:

# zpool status

  pool: zroot
 state: DEGRADED
status: One or more devices has been taken offline by the administrator.
        Sufficient replicas exist for the pool to continue functioning in a
        degraded state.
action: Online the device using 'zpool online' or replace the device with
        'zpool replace'.
  scan: none requested
config:

        NAME                      STATE     READ WRITE CKSUM
        zroot                     DEGRADED     0     0     0
          raidz2-0                DEGRADED     0     0     0
            da0p4                 ONLINE       0     0     0
            da1p4                 ONLINE       0     0     0
            da2p4                 ONLINE       0     0     0
            da3p4                 ONLINE       0     0     0
            12385660660938982245  OFFLINE      0     0     0  was /dev/da4p4
            da5p4                 ONLINE       0     0     0

errors: No known data errors

Check partition scheme on an existing disk:

# gpart backup da3

GPT 152
1            efi         40     409600 efiboot3 
2   freebsd-boot     409640       1024 gptboot3 
3   freebsd-swap     411648    4194304 swap3 
4    freebsd-zfs    4605952 1948852224 zfs3

Configure partition on new disk by copying it from an existing disk. Following command will copy partition table from an existing disk (da3) to new disk (da4):

# gpart backup da3 | gpart restore -F da4

And verify partition table on the new disk:

# gpart show da4
=>        40  1953459552  da4  GPT  (931G)
          40      409600    1  efi  (200M)
      409640        1024    2  freebsd-boot  (512K)
      410664         984       - free -  (492K)
      411648     4194304    3  freebsd-swap  (2.0G)
     4605952  1948852224    4  freebsd-zfs  (929G)
  1953458176        1416       - free -  (708K)

Now replace the disk in zpool:

# zpool replace zroot 12385660660938982245 /dev/da4p4

Make sure to wait until resilver is done before rebooting.

If you boot from pool 'zroot', you may need to update
boot code on newly attached disk '/dev/da4p4'.

Assuming you use GPT partitioning and 'da0' is your new boot disk
you may use the following command:

        gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da0

If this is your boot volume, make sure the new disk also has boot code:

# gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da4
partcode written to da4p1
bootcode written to da4

Your zpool should be now resilvering the new disk. It may take a few hours:

# zpool status

  pool: zroot
 state: DEGRADED
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
  scan: resilver in progress since Wed May 20 23:05:00 2020
        920M scanned out of 1.84T at 13.5M/s, 39h32m to go
        153M resilvered, 0.05% done
config:

        NAME                        STATE     READ WRITE CKSUM
        zroot                       DEGRADED     0     0     0
          raidz2-0                  DEGRADED     0     0     0
            da0p4                   ONLINE       0     0     0
            da1p4                   ONLINE       0     0     0
            da2p4                   ONLINE       0     0     0
            da3p4                   ONLINE       0     0     0
            replacing-4             OFFLINE      0     0     0
              12385660660938982245  OFFLINE      0     0     0  was /dev/da4p4/old
              da4p4                 ONLINE       0     0     0
            da5p4                   ONLINE       0     0     0

errors: No known data errors

Wait for it to finish. You don’t need to manually take the disk online.

It should be back to normal after a while:

# zpool status zroot
  pool: zroot
 state: ONLINE
  scan: resilvered 295G in 6h59m with 0 errors on Thu May 21 06:04:08 2020
config:

        NAME        STATE     READ WRITE CKSUM
        zroot       ONLINE       0     0     0
          raidz2-0  ONLINE       0     0     0
            da0p4   ONLINE       0     0     0
            da1p4   ONLINE       0     0     0
            da2p4   ONLINE       0     0     0
            da3p4   ONLINE       0     0     0
            da4p4   ONLINE       0     0     0
            da5p4   ONLINE       0     0     0

errors: No known data errors