Somebody mentioned that removing CS6 in this thread fixed it and then they where able to install older versions again and the association stayed with CC2018
I was having exactly the same problem, I need both Photoshop CS6 and CC installed (I'm a freelance software trainer), as some of my clients don't have a Creative Cloud subscription. All was fine until the other day when my Mac running Sierra (10.12.6) suddenly decided to open CS6 whenever I double-clicked a psd, .tif, .jpg etc. file in the finder, or used InDesign's Edit Original command (from a right-click shortcut on a selected image). Very frustrating as it had been fine. I tried the usual, Finder 'Get Info' window > Open With… > Change All method - to no avail, it just kept reverting back to CS6. So I checked out some of these forums about installing and uninstalling.
I decided to unistall CC, and reinstall. That didin't help. So I unistalled CS6, leaving only my newly installed CC2018.
NB If you need Photoshop CS6 Don't uninstall CS6 unless you have an installer for it as you can no longer install CS6 using the Adobe Creative Cloud App, as you can only go as far back as CC 2014 (I still have my original CS6 DVD I got before I moved to a CC subscription, so luckily I was able to reinstall CS6)
After unistalling CS6, I checked that the file association was back to CC. Then reinstalled CS6 from my DVD, but using my CC subscription credentials, rather than the serial number (as I was too lazy to key in my long serial number!). That has seemed to cure the problem.
HOWEVER - When I went to my LaunchPad (which I use all the time) to Launch CC2018, the CC icon had been replaced by the CS6 icon. Grrrrr... So to get around that, I uninstalled CS6 again, (the CC2018 icon reappeared in my LaunchPad) then reinstalled CS6 inside a folder in my Applications Folder called Photoshop CS6 Folder, so the installed Folder is down one level inside it. That kept Photoshop CC2018 as the icon in Launchpad. (I may not have needed to re-install it, I may have got away with just moving it inside another folder inside my Applications folder)
So to summarise...
In your Applications folder, create a folder called "Photoshop CS6 Folder" or similar,
Re-install CS6 from your CS6 installer DVD
Make sure that when you re-install select the /Applications/"Photoshop CS6 Folder" you've just created, rather than using the default /Applications folder, if - like me - you want to see the CC icon in your Launchpad, not the CS6 one.
I hope by sharing my experiences, I can help you and others out with this very annoying issue.