First, make sure you are in the Wallet / NFT tab.
Once there, locate the wallet that you wish to remove and tap on the 3 dot ( ⋮ icon) next to it.
In the dialog that pops-up, tap the Remove button.
A prompt will appear informing you that you should be able to reimport a wallet with the 12-Word Recovery Phrase that you have written down.
Please note - If you don't have the Recovery Phrase of that wallet saved somewhere safe, you won't be able to import it back later and you will lose all funds & assets! It is highly recommended to backup any wallet that you have funds in and don't want to lose.
If you are certain that you wish to remove the wallet, tap the Remove Wallet button

After tapping the Remove Wallet button, you will be returned to to the Wallet / NFT screen, and the wallet you removed will no longer appear; however, you can always import or watch the wallet again by following this guide: Importing a Wallet

Note - you can also remove a watch wallet.
If you want to add the removed watched wallet again, you only need it's Public Address.