Lightroom 6 can be purchased as a standalone product on the following website:
1. Scroll to the bottom of the page.
2. Click on the Buy button next to Lightroom.
3. Indicate that you want to purchase an upgrade and indicate what version you are upgrading from. The price will change.
4. Complete your purchase and download and install Lightroom 6.
But I don't understand why you want to pay $79 to purchase an upgrade. Lightroom CC is precisely the same program. And it's included as part of your creative cloud subscription plan. What do you expect to gain by purchasing an upgrade in addition to what you have in the cloud plan?
I'm really just trying to upgrade to 6. I currently work with the stand alone version as I had that before I signed up for CC. So in order to take advantage of the LR CC upgrade to 6 (which as you say I've already got access to) I'm wondering how to get it without messing up the existing stand-alone install. If I try to download the CC LR will this "intelligently" replace the stand-alone set-up?
No. Installing Lightroom CC will leave your standalone Lightroom 5 completely intact. However, after you install Lightroom CC and start it, it will find your existing catalog that you have been working with and will offer to create an updated copy for Lightroom CC. You should choose to do this because it will take all of your work that you have done and duplicate it in a new catalog format and will add the suffix of -2 to the name of the catalog. From that point on Lightroom CC will use that new catalog. If you want to keep Lightroom 5 you can do so. However, I have chosen to remove it.
If you go ahead and purchase Lightroom 6 as an upgrade, it will do the same thing. It is exactly the same program and will need to create the same updated catalog structure. In my opinion the only thing you are accomplishing by purchasing Lightroom 6 is getting rid of $79 that you don't need.
I think I should also emphasize that if you choose to upgrade, as you put it, your Lightroom 5 you are not doing anything to the Lightroom 5 program. Upgrading does not utilize any part of the Lightroom 5 program files. Lightroom 6, even when purchased as an upgrade, is a complete new program. It will be installed in addition to your Lightroom 5, and will utilize the user files that have been created (i.e. your catalog and previews, etc.). But there is nothing special about an upgrade that utilizes any part of the older Lightroom program files. Each installation of Lightroom, regardless of whether it's an update or an upgrade or even a new installation, is a complete new program.