One should really have proper tools to remove the bezel assembly from the watch head. I did succeed in removing a bezel assembly from a Hulk using a case knife and some hard flexible plastic pry tools, but trust me you don't want to go there.
Once it's removed, the nylon gasket that fits in a groove in the bezel is visible. Check it for uniformity to make sure it's not been deformed. It can be lightly sanded to restore its proper shape if necessary. This is the item that must be reused with a gen bezel assembly if you do not also get one with the gen assembly. One member documented having to cut out about 1/8 inch of the rep nylon gasket to then use it with the gen assembly. See the linked post below.
Make sure you press the bezel assembly back onto the watch head BEFORE affixing the new insert. Pressing an insert is a sure way to crack it.
If you are just replacing the insert and not the bezel, you CAN try to pry it up with the bezel still installed. Applying heat will help loosen the adhesive. Apply the heat from the side, aimed up and away from the case as much as possible. Heat will dry out watch lubricants. The insert is held in with a ring of strong double sided tape. You can buy these tape rings from WSO on ebay to adhere the new insert.
https://forum.replica-watch.info/forum/modification-repairs/7195798-gmt-master-ii-bezel-replacement