If this is a deliberate choice, then the installer should make this clear. Rather than just refusing to install, it should inform you that M2M is already installed, and that you should use the built-in updater.
What about if M2M gets corrupted, and you can’t use the built-in updater? I have had a situation where GCC died. As far as I can tell, there is no way to repair an M2M installation, either from within M2M studio, or using the repair option from the add/remove programs control panel item (because M2M doesn’t have one). It is therefore necessary to first remove M2M, then run the installer again. However, as has been mentioned before in this thread, it is not obvious how to uninstall M2M Studio. Apparently M2M itself can just be deleted, but other things installed along side M2M have uninstall batch files that must be run.
There seems to be a lot about the install/uninstall process that assumes you have detailed knowledge of the inner workings of M2M Studio.