[The Yeti]

Assuming its a glue in type it's not rocket science

Pop the corner up I used to use wire as a cuter so you just run some through the bit you poped up and get some one on the inside and run it round the adhesive till you've done a lap then lift it out

Clean the old sealer off and put a bead of sikaflex around (a good healthy bead say 1/2 inch sort of thing then drop the new one in ensuring that it doesn't come in to contact with the metal (A Pillars roof and fire wall) and space it evenly so the surround will go back on and not look stupid (a couple of bits of vacuume line does the trick until the sikaflex sets)

DON'T use generic silicon as this promotes rust

Probably wouldn't hurt to phone around as the price for a windscreen bloke to come out and stick in a new one isn't that bad (windscreens obrian will be the dearest you can find)

Also wear a P2 dust mask & eye protection when you do this as you pull the old one out you can get fine fragments of glass dust and that does all sorts of messy stuff to your eyes and it's believed that this same fine dust will become the next asbestos