Hi yes I agree with davyboy go to a garage, the windscreens are bonded in and the "professionals" usually cut them out or break them to remove them, the fitting and bonding of a windscreen has got to be exact with no variations and success depends on 100% correct preparation of all surfaces and the adhesive product use and cure time if not
you will fail!
If you are determined to do the work yourself the best windscreen adhesive product is 3M Windo-Weld Superfast Urethane (as used by the professionals), you should use this in conjunction with 3M Glass Cleaner and 3M Single Step Primer.
All of these 3M products are available on Amazon but they are not cheap and the following is the product codes:
3M Windo- Weld Super Fast Urethane Part number: 08609
3M Single Step Primer Part number: 08681 125ml Bottle - Approx 25 installations
08682 30ml Bottle - Approx 6 installations
3M Glass Cleaner part number: 08888
All these products have a shelf life of only
9 months and you must use them within
7 days of opening the product, data sheets and comprehensive instructions on the products are available from 3M on the web and also there are tutorials on YouTube!
Good luck!
Andy