There are two possible scenarios.
- You do a fresh install of Umbrel
In this case, just add Knots through the community store UI (section if you don’t have a node yet in the docs). Afterwards, you’ll have to install any additional apps that depend on Bitcoin through the command line.
- You have a working setup of Bitcoin Core + apps that depend on it
You will have to use the command line to replace Bitcoin Core with Knots, as outlined in the docs, section if you already have a node. You don’t have to reinstall any apps that depend on Bitcoin Core. Just leave the rest like it is. They will use Knots instead of Core automatically afterwards.
I can personally recommend going with option 1, because it is the safest route. Replacing Core with Knots on a running setup is scary to be honest and if something goes wrong, you will have a bad time. On the other hand, if option 2 works out for you, you don’t need to do an entire resync of the blockchain.