As if I didn't already know this. Please. Maybe my uses with 8.1 were unluckily the worst case scenarios but I have really struggled with this OS with very simple things.Right-click file, select "open with", like you've been able to do since...hell, I don't recall. Many years though. You get a window with choices windows believes can handle the file format in question, pick your program, tick "always use this program" checkbox or whatever the actual text for it is, if you want your choice to be made permanent.
HTH.
W10 is such an improvement from 8.1! For my use cases anyways.