how to convert from the latest MacOs to a previous version?

To answer your question: you can only revert if you erase your hard drive completely and then reinstall (restore) from your Time Machine backup and/or clone which you made before upgrading. You do have a backup/clone?

Good Day;

QuickTime Kirk's response to you is not in the least helpful. Many users were not aware of the ramifications of OSX becoming exclusive 64 bit. Many software programs for the iMac have not been upgraded to 64 bit and will not be. I have not upgraded for this reason. Others have needed the process to downgrade to their previous operating system. Their are few posts on how to do this. Here is one such link https://support.apple.com/en-ca/guide/mac-help/mh15216/mac I hope this is Helpful for you.

Regards

FWIW, QuickTimeKirk's answer is the best possible answer. If you were not aware of the 64 bit transition that has been coming for over a decade I'm afraid you have not been paying attention and your advice to revert is not the best option.

Perhaps if you were following the post more closely, you would realize that I was aware of the 64 bit transition, others were not as per the many posts relating to that. Just because you and QuickTime have a high degree of knowledge as indicated by your Level scores, many users have no computer background other than operators of software programs. LIUJIN asked for assistance, I attempted to answer his question. Shouldn't you two try to help as you obviously have in the past instead of admonishing him/her? You must be aware that there are many software programs that are 32 bit and will not be rewritten for 64 bit, and some requiring costly upgrades such as MS Office?

Revert your Mac to a previous macOS version

But as QuicktimeKirk said, it would be better to upgrade your apps to 64 bit.

Update the incompatible software.

Two years of notifications from Apple is not enough to make you aware?

Apple software has been 64 bit since 2011.