Catalina on iMac

I want to get rid of Catalina and I am NOT confident about doing it can it.

Can it done through CCleaner or can anyone help. I am not that confident, but my computer is so slow and everything I read tells me the problem is Catalina

That is not something that CCleaner could do.

If you don't like Catalina you could try upgrading to Big Sur:

https://support.apple.com/en-gb/HT201475