Removing Apples APFS Filing System

I have just updated my operating system from OSX 10.11.6 El Capitan to OSX 10.15.4 Catalina. I wish to remove this and go back to El Capitan. I can’t do this because the new OSX uses Apples APFS Filing system. I can’t reformat the hard disk back the HFS+ because I can’t unmount the internal drive. Can somebody advise me if there is a way around this problem please.
Regards Salicet.