How can I undo an upgrade of Reminders?

My iPhone is backed-up daily. The Reminders app is working fine (updated) on my iPhone. The problem is that my Mac was not updated and all reminders seem (?) gone from that device. I don't intend to upgrade past current Mojave, so I just want to go back to the previous version on my iPhone so the app will work on all 4 of my devices.

OK- so I like your answer above. Solves my problem until I'm ready to upgrade my MacBook Air! So, now another question: Is there any way to re-order my Reminder Lists? I have many (I'm sorta compulsive!) and I have to keep scrolling through the ones I don't use much to get to those I use frequently...I'd like to place the frequently used lists up top.