It’s simple:
- in Umbrel dashboard - Lightning - on right top 3 dots - backup channels
- download that file into a safe place together with your 24 words seed
That is enough to restore the funds, following the procedure from FAQs
You install a new Umbrel instance, use the 24 words seed to restore your LND wallet and then using the instructions, with the channels.backup file will close the channels and funds returned in online wallet. Done.