Just make sure you have all the nuts off, and if the sealant is the original butyl seal, it will pop off with just some minor pressure. Over time the butyl loses its stickyness, and usually the window leaks, requiring replacement of the seal.
Buy 1/4" butyl ribbon (the 3M stuff is best) to re-seal it. if you use the stuff in a caulk tube (urethane windshield adhesive), it will seal tight but it may never be removeable again. One strip of butyl ribbon is enough to do both windows if you just apply it the same way as the factory stuff, you'll see how to do it with the ends on the bottom of the window when you pull it off.