I am getting daily popups in the AM when I restart the PC that "I am now running 5.87.9306". A few days ago, CCleaner correctly updated to 5.88.9346. Ever since then, I am getting these popups daily. It's as if I had never updated even though the correct version number appears on the app.
Odd. (What happens if you click on 'Check for Updates' in CCleaner? It should tell you that CCleaner cannot be updated).
The best I can suggest is that you re-install v5.88.
Doing that should correct any registry entries that may be saying you still have v5.87.
Download the 'Slim' installer from here (3rd one down) and double click it to re-install straight on top of your existing v5.88. ie. no need to uninstall first.
It installs the same CCleaner, it's just a bit quicker on the install. (Because it misses the adverts out).