I'm not sure you can do that from a form. You can do it from Access via the FILE tab, but that is not on the menu or ribbon.
I think if you want to do a backup, Access has to momentarily close the file to copy it - because MSACCESS.EXE is running as its own process and then your app is running as a second process. (You can verify this through Task Manager, which will show that Access has a child task which is named for the app file.) Anyway, my point is that if you wish to back up the app file, it cannot be open which means that form cannot be open to host a button.
I could be wrong, and would be willing to believe I am wrong - but at the moment I don't think I am.
However, did you literally mean "backup the database" or were you talking about another entity WITHIN the database being backed up. That would make a difference.