Don't have the commands to hand but basically stop witness service on both arrays, remove witness server on both arrays and then follow the add witness server routine for the new witness.
The steps/commands for each stage are in the remote copy guide.
Based on some rough notes I have it kind of goes like this (remember to do one step at a time, on both arrays, can use the -remote option sometimes but I prefer having 2 ssh sessions so you can clearly see the progress with the show command as you go. Can take a little while for each command to complete in the background, the cli comes back quick so do a couple of shows to see it finish)
.
Code:
showrcopy -qw targets
setrcopytarget witness stop <target_name>
setrcopytarget witness remove <target_name>
setrcopytarget witness check <IP of Quorum Witness>
setrcopytarget witness create <IP of Quorum Witness> <target_name>
setrcopytarget witness start <target_name>
setrcopytarget witness start –remote <target_name>