You need to remove the mount to install the battery or to change from high mount to low profile mount.  Use the small Allen or Torx wrench to remove all four screws. Set aside the screws. Install the battery or change the mount as needed.  Carefully place the mount onto the scope and then reattach the screws so that they are barely hand tight.   Now tighten the screws gradually a bit at a time, first one screw, then the one diagonal to it.  This is to ensure the screws are evenly tightened.  You should hear the screws "squeak" a bit when then are tight.  For a new optic, if you have the time, let the scope sit overnight and re-tighten the screws the next day.  As the rubber gasket that protects the battery may compress a bit.   You can also use a low strength thread locking compound like Locktite Purple(tm).  Loctite or a similar thread locker should be used with any of the Holosun sights.