Lets clear a couple of things up first.
1, You have the Photographers CC subscription which gives you access to PSCC and the current version of Lightroom.
2. PSCC does not include Lightroom.
Now what you will need to do is delete the Lightroom app on your iMac and restart the Mac. Then you can d/load Lightroom from the CC app.
Just as Geof says, and so you are aware when you "trash" the existing Lightroom Application it will not interfere with your LR Catalog, Preferences and Personal Files.