Hi John. What you are proposing to do should be similar to when I upgraded from LR4.4 to LR5.x, hence me chipping in here. When you have downloaded LR6.6.1, you will have both a LR5 AND a LR6 icon in Launchpad in the Dock of your Mac. Just open LR6.6 and you should then get prompted to 'convert' your LR5 catalog to LR6. Make sure that you have backups of the catalog in Time Machine etc.
A couple of other steps that I would personally do would be to click on 'Updates' once you have installed LR6, to ensure that you get onto LR6.6.1. If that is the version that downloads, doing this will confirm it for you.
I would also take the opportunity to download Mac 10.11.6 'El Capitan', which I did myself yesterday...my reasoning being that the next OSX is not far away, & I know that El Capitan works fine (now !) with LR5 and LR6, so it should be fine for you for the foreseeable future - this might not be the case if you stay on Yosemite, as Adobe release further versions of Lightroom, which may contain features that you would appreciate.
Cheers, and I hope that this helps.