To change your Live MT4 password on your Android device, you need to:
Tap on the menu opener on the top-left portion of the MetaTrader application.
Tap on Manage Accounts.
Tap on the properties icon on the top-right portion of the application.
Tap on Change Password.
Select Change Master Password, input your current Live MetaTrader account password and input your new desired password twice to confirm this change.
Tap on Next.
A pop-up window will confirm your change.