How do I clean fly screens when they’re stuck in the window frame?

For screens that cannot be removed I would recommend using a soft round upholstery brush that connects to the end of your vacuum hose. If you don’t have one you can purchase one from just about any store that sells vacuums and accessories. This way you can brush and vacuumed the dust, dead bugs, cobwebs etc from each screen. It’s easy, quick and will do a great job.