Replacing triangular vent window

[chbrow10]

I'd like to replace the vent window in my 1999 Islander 268. This is the one near the corner that opens like the old cars of the 60-70s Does it just slide in there? How is it retained? Do I have to take apart the window frame?

 

[HMBJack]

Yours may be like mine. It is simply silicon caulked in place in the U shaped frame.
No need to take apart the window frame. Just carefully remove it from the U shaped frame.
Heating it a bit with a heat gun or hair dryer will soften up the silicon caulk and it'll slide right out.