Get something like this:
https://www.amazon.co.uk/DOUBLE-SIDED-ACRYLIC-ULTRA-metres-x/dp/B00H4F7PB6/ref=sr_1_45?ie=UTF8&qid=1481828947&sr=8-45&keywords=double+sided+tape
To re-fit:
Start the car and go for a 20 minute drive, get it warm.
Then:
I'd peel them off carefully, remove the old tape from them with a single sided blade (cut towards your chum not your thumb) once removed, clean any residue with white spirits. Then while they are off, rub down all the sharp edges with 600 wet and dry paper (use it dry) then wipe down again with the spirit.
Next apply the fresh tape and trim with the blade if needed. With the backing on, dry fit and bend to shape where needed so they stay in place/shape without the tape holding them.
Next, clean the vent with the spirit, dry off with a microfiber (don't just let it flash off) peel the backing tape and apply to the vent starting at the bottom and firmly press into place.
Wipe over with the spirit to remove greasy paw prints, then fire the car up again and go for a drive, get some heat through the vents and see how they are. Take the microfiber with you to apply pressure if they lift a little.
If they don't, then you can sit back and admire your work