Well, something in your light path is very dirty. It's not possible from the image to tell which surface(s) that might be. You can try limited experiments from the outside to figure it out. For example, try rotating the camera. If the dirt does not rotate with the rest of the image, then hooray, it's on the camera lens, and you can clean it.

If it's somewhere in the guts of the microscope, it's not advisable to try to take it apart without experience. You'll have to find someone who knows this kind of microscope to fix it for you. And use a dust cover in the future to minimize this problem.

Try with GIMP: https://www.gimp.org/downloads/