If you know your current password, you can change it directly through the MT5 trading software:

Desktop (Windows / Mac)

Click "Tools" → "Options" → "Change" in sequence, enter your current master password, and you can then modify the master password or read-only password (changing the read-only password will not affect the master password).

Mobile (Android)

Click the account for which you want to change the password, select "Change Password" and follow the prompts.