To change passwords, you need to access the "Data and management of TA" section, select a trading account and in the "Management" section click on the icon "blue circle" opposite the password you want to change:
In the window that appears you need to type and confirm your new password, then click on the "Update" button:
The password will be changed instantly. After changing the password, it is necessary to relogin in MT4 (re-connect to the trading account using the new password).