As soon as I go in Uniswap, it already loads the Metamask wallet that was used previously... but I do not want to use that Metamask wallet, I want to use a different Metamask wallet. How my I do this, please?
You can disconnect a Metamask account from a dapp by clicking on the three dots menu on the top right (next to Account NN).
Select "Connected site" it will show a dapps list that were authorized to use the current account. You can remove the dapp from the authorized list with the trash can icon.
To authorize an account to use a dapp you have to select an already authorized account, then switch to the unauthorized one, a popup should appear asking for confirmation.