@gychang - unfortunately the included version of Brisk Menu doesn’t support changing key bindings - the fix for that will be in for 17.10. I know @Wimpy maintains a PPA that has the latest version, which supports changing it. You can try that one:
Alternatively, you can use the Advanced MATE Menu, which supports changing the keybinding in the Preferences.
All that said, pressing Alt+F1 should still pop open the main menu wherever your mouse cursor is, as far as I know, so that may still be an issue on your side?