Downloaded CCCleaner for PC, although I have a Mac computer. I was led to download CCCleaner for PC because the link in the mail from CCCleaner took me there and not to the Mac part.
What to do ? How to return the CCCleaner for PC and replace it with the CCcleaner for Mac ?
Are you saying that you have paid for a licence for the PC version by misake?
If so you will need to contact support by email at - support@ccleaner.com
They will be able to change it for you to the correct one for a Mac.
If you just want CCleaner Free for a Mac then you can download it here: https://www.ccleaner.com/ccleaner/download?mac
If you actually bought a Mac key, but just got the wrong download, unlikely if it was in the email, then you can download that Free Mac version and go to Options >About> 'Upgrade to Pro' and enter the key there to register it - Note that a PC Pro key will not activate Mac Pro, the keys are different. (Unless it's Pro Plus).