Not sure when reading your message, if you mean  the full  outside frame or just the plastic window, if it's the  frame remove inside finishing frame fixings    remove and there should  be screws holding the outside frame,  looking at a pic from van roof to  window frame looks as if there is gaps, it should be non setting  you can buy it on a tape, or buy a tube of it,  but not everybody uses non setting,  or is it the black seal, and if so is there not fixings on the two hinges on the outside, as previous message said more  pics would help, and a reply,