Let's go through the process:
> Tap the 3 horizontal lines on the MetaTrader application.
> Tap on Manage Accounts.
> Tap on the Account Settings icon.
> Tap on Change Password.
> Choose Change Master Password in the first selection box.
> In the second selection box, enter your Current Master Password.
> In the third and fourth selection box, enter your New Master Password.
> Tap on Next to continue.
> Tap on OK once the finalization box appears.