How do I remove a device I no longer use?

Settings > Account lists every registered device. Remove the one you are retiring: the slot frees up immediately (useful on the free 2-device plan), and the removed device is signed out the next time it tries to sync. It stays visible under "Recently removed" for 72 hours so you can confirm it is gone.

One honest caveat: removal is not a security boundary on its own, because anyone holding your recovery phrase can sign in again. If a device was lost or stolen, the phrase is the thing to protect, and the app lock (PIN or biometrics) is what keeps a found device from being an open door.