You can order it from any aairstream dealer and have it shipped to you. The windows are hinged at the top and simply rotate up until the "J" clip at the top disconnects from the aluminum frame around the window. Installation is the reverse. Oh, you do have to disconnect the arms on each side of the window. The last time I checked it is about $200 for one of those larger windows.
Craig and Carol
1997 34' Excella 1000
1970 27' Overlander, International
2009 Ford F150 5.4L
ProPride hitch with 1400# bars
Past President Southeastern Camping Unit (12)