1. Right click on any *.blend file, choose Open With > Choose another app

Seems like you encounter the dumbest design the windows registry system can potentially have. If the executable name is the same one (.blend here), it will cause some bug and not updating the table correctly.

2. Ignore that Blender App and choose More App > Search on this PC, also check "Always open .blend with this app".

You can make your blender 2.79 executable named as blend279.exe. This will let the register miss the original 2.79 exe and update the new value after choosing new blend file.

3. Choose the 2.80 version of blender.exe inside your install folder

example below to make it "forget" 2.83 blender:

4. Done.