Setting Photoshop as the default app

I have Photoshop ver 8 on a  windows 10 pc.  I am not able to set Photoshop as the default app. On the default settings window the settings square contains a plus sign and the box appears to be blanked out. I can get to the pull down list of the available apps in this category. When I click on Photoshop nothing happens. I'm stuck on the 3D paint shop app. How can I correct this?  Tnx