unless you need the old editor, delete it. It should then open in iReal Pro.
Or if you click once on the file to select it, then cmd-i (apple-i) for the info on that file, you should be able to select iReal Pro in the Open with section.
Here is an Apple support link about this:
https://support.apple.com/kb/PH14279?locale=en_US

I hope this helps.