You can change the trading password and the investor's password on the Demo account created through the MT4 terminal or your personal cabinet (in some cases - only through MT4).

To do this via the MT4 terminal, you need to launch MT4 and select the necessary demo account in the navigation window and right-click on it. Choose "Change password" in the appeared window:

Then the "Change password" window will open: 

where you need to specify the trading password, tick the "Change investor's password" box, enter the new investor's password and confirm it.