If you have multiple accounts in MetaMask and need to switch from one to another, please follow this procedure:
MetaMask Extension: Click on the round favicon icon in the top right corner. You will find a list of accounts; click on each of them for details.
It can be useful to rename them in order to avoid confusion.
MetaMask Mobile app: click on the 3 horizontal lines icon on top left of the app. Click on the drop down arrow next to "Account", and choose from the list of accounts the one you would like to use.