undo operating system update

While it cannot be done on your iPhone, it is possible on the Mac, but not easy and there may be some data loss. Review the information provided here to see if that is a task you want to attempt.

https://www.macworld.com/article/671318/how-to-downgrade-macos-revert-back.html

Our friend, @Mac Jim ID, provides a good link to the process. 👍🏽

However, imho, it would be much better to work through any issues you may be having than to revert to an earlier OS and then leave you Mac locked in to that older OS for fear of having to learn something new. Too many user find themselves falling behind the OS upgrade curve and then try to catch up after too many years. This limits the practical, productive lifespan of the Mac in the long run.