Examine the inside and outside closely to compare them...one side should have an extra 'hump' in the middle that runs parallel to the whole gasket all the way around. Ours is on the outside, presumably to keep kiddos from pulling it out to relieve boredom and frustration during their bus ride. If you find that, follow it til you find the seam where it is joined in the round. Use something flat to pry it out of its groove. The window *should* be pushed toward the side that has the now vacant groove. I found these instructions in the owner's manual for my BB.
the process will be microscopically easier if its nice and hot.
Inside on our bus..no extra 'hump':
Outside on our bus...removable gasket piece in place to hold window in:
Outside our bus...locking piece removed about an inch...see the groove? It will stretch when you remove it so be careful not to snap it. Move your grip every few inches so you're not pulling it from a foot away.
this will give you a basic understanding of how the gasket works.