How does one replace a rear view mirror on a 2000 S-Type the mirror is off but cannot figure how to get the clip back on to secure the new mirror?

If the button(metal tab on windshield) is off there is a glue you can buy to reglue it. After following the directions on the glue package take youe mirror and slide it from bottom up using a strong even force while wiggling it into place, also keep an eye on any electrical wiring which needs to stay in place(ie rain sensor and auto dim mirrors) professional techs break about 1 out of every 50 windshields during this repair so be careful.