...or any other app for that matter.
This is a Windows 10 issue, not Flash Player.
I found the following Windows 10 support document https://support.microsoft.com/en-us/help/4028161/windows-10-change-default-programs If this doesn't work, please contact Microsoft for assistance.
Even trying this doesn't work for me.
You'll need to contact Microsoft for assistance. This is a Windows issue, not Flash Player issue.