To change a master password ("Safe Combination"), first open up the database whose password you wish to change. Once opened, go to the Manage menu and select Change Safe Combination. You should enter the current master password, and then the new master password (twice, for confirmation). The master password can be entered using the physical or virtual keyboard.
Password Safe has a built in check for what it considers to be weak master passwords. In general, a master password (like any password) should consist of mixed case characters with at least one number or special character (such as those found above the number keys on a keyboard). If a master password is entered that the program considers weak, it will prompt for confirmation. You may the option to accept the originally entered (weak) password, or to enter a new one.