1. From Home, tap your profile picture in the top left corner.
2. Scroll down and tap Switch Account.
3. Swipe left on the account you want to remove.
4. Tap Remove > Remove.
Android:
1. Form Home, tap your profile picture in the top right corner.
2. Scroll down and tap Switch Account.
3. Tap the three-vertical-dotted icon next to the account you want to remove.
4. Tap Remove account
5. You may be required to confirm the removal command. If that is the case, tap confirm, and your account will be removed.
See: My Blocked List on Messenger – How to Unblock People on Facebook Messenger