If using iOS:
Open MT5 and go to Settings.
Tap the arrow next to the account to display all the accounts logged in on the app.
Tap on any account to log in.
MT5 will now be logged in with that trading account.
If using Android:
Select Manage Accounts from the main menu.
Tap the trading account you wish to make active in the Accounts tab, enter the trading account’s credentials if prompted, and then Sign In.
MT5 will now be logged in with that trading account.