Interior rear view mirror fell off my 98 BMW 540i

There is a small set screw that holds the mirror to the small metal button. Most vehicles use a small Phillips screw but some use a Hex Key Wrench (Allen Wrench) type screw. Slide the mirror off the button. Make sure not to remove the glue before you put the button back on. Trace around the glue with a grease pencil on the outside of the windshield before you remove it. So you know exactly where to replace the button. You can buy a Rear View Mirror Adhesive Kit for around five dollars.

THE METAL DISC NEEDS TO BE REGLUED TO THE WINDSHIELD TURN THE DISC WITH A PLIARS IT WILL UNLOCK FROM THE MIRROR THEN GO TO AN AUTO PARTS STORE AND BUY THE CORRECT GLUE
PUT A PIECE OF TAPE ON WINDSHIELD BELOW WHERE THE MIRROR FELL OFF THEN TAKE A RAZOR BLADE AND CLEAN THE WINDSHIELD AND METAL DISC USE THE GLUE KIT AS INSTRUCTED WHEN YOU GLUE THE MATAL PIECE TO THE WINDSHIELD PUT A PIECE OF DUCT TAPE OVER THE METAL DISC TO HOLD IT IN PLACE WHEN DRY TAKE MIRROR AND TURN IT SLIGHTLY TO THE SIDE SLIP IT OVER THE METAL DISC THEN TURN SLOWLY TO THE DISC LOCKS IN PLACE

Just put some good glue on it and stick it back up there the way it is. but the metal mount should slid out