sunroom window repair

The sunroom on my house was there when I bought it. I think it is a Four Seasons brand, but am not sure. It is aluminum frame construction with glass windows except for the curved pieces which are plexiglass or Lexan. The plexiglass is now all broken up due to age. Does anyone know how to replace the plexiglass on one of these? I can't see how to get it out. Many thanks for any help here. Here are 2 pictures of it.

John