Well...firstly remove the grips.
Underneath the grips right behind teh trigger frame, going straight up, is a hole...in teh hole is a large grub screw...unwind this a touch as it will aid installation...this screw is what the magnets act on.
Look at the pin on the trigger...it may have one side larger or knurled...knock it out from teh other side with a hammer or suitable object and a drift...or blunted nail...be prepared to have to go ape at it...put new trigger in, pin back in, bobs your uncle, one scratched trigger frame..or maybe not