If you are not so familiar with command lines, then just shut down your node, take the external drive and plug it into your Mac.
Browse to that folder
umbrel/umbrel/lnd
and copy the backup file (from your local desktop/PC/whatever folder you have it) and rename it to be channel.backup
That’s it.
Then run from SSH
cd ~/umbrel && ./bin/lncli restorechanbackup --multi_file /data/.lnd/channel.backup
After you run this, wait for 1 minute. You should now be able to see your channels being closed on http://umbrel.local/lightning .
You should wait patiently until the funds are coming back to your onchain wallet. It will take at least 40 blocks. You can see/check the details of closing channels in the troubleshooting guide