You need to reset the file association. Right-click on one of the files, and choose Get Info. In the next window, go to Open With... and navigate to Finale 26. Set this to open all similar files.

Thank you so much!  Should I choose "change all?"

>Should I choose "change all?"<

If you want all Finale files to open in 26, yes.

You may have to to this with both a .musx and a .mus (if you have files that old) to automatically open everything in 26.

Is there any reason NOT to open all files in 26?

And, why would a file created in 26 open in 2014.5, and not 26?

A 26 file would open in 2014 if you have 2014 selected as the default app to open .musx files.

Many/most/all programs automatically update the file associations. Much to my surprise (not) Finale doesn’t seem to do so.

Many apps replace or overwrite the older version when updating. Those that don't often exhibit this behavior. 

It's no big deal and easy to change as you saw. This has been 'Mac Owner 101' for over 30 years.

With Finale, there can be another issue. When file suffixes came into being (late OS 8 to OS 9). In addition to Finale, MusicTime, MusicTime Deluxe and another app also used .mus as an extension. If you have those old files, you have to select the correct app for every file—if you have a modern app that can read them.

Thanks for your prompt reply.  My other programs always would overwrite the old file and make it "current."  Never knew this had to be done.  I have been a Mac Owner for over 30 years.  I must have gone out to recess when the teacher covered that!  Again, thanks for the help!