Lightroom 5 is still the default handler for file type LRCAT.
Open Lightroom 6 and use the Open command and navigate to the catalog.
If you are going to leave Lightroom 5 active on your system, you may need to change the OS to autolaunch LR6 instead of 5 when you double click an LRCAT file.
You're likely still running LR5. LR6 installs side by side, so LR5 is still there.
On my Mac, I see the new version 6 as "Adobe Lightroom". The old one is "Adobe Photoshop Lightroom 5".
Thanks. I finally figure it out. All my old catalogs are now being upgraded