Maschine + Plus freezes after update . How to downgrade?

Perhaps upgrading firmware to latest version 0.1.9 may help

How-to-Update-the-Firmware-of-Your-NI-Hardware

I suppose you need to connect M+ to computer with M+ power cable disconnected to get into controller mode in your case

From time to time i face similar problems with my projects too. What helps in my case:

1. Just wait several minutes, M+ will reboot. Try loading the project, it may load this time. Repeat several times in case of failure.
2. Or try loading different (smaller) project or several ones. After that problematic project usually loads fine.
3. Or rollback to one of previous versions of the project, redo the same steps to achieve final result and then save the project with a new name. This new version can potentially be loaded just fine without any issues
4. After successful load of problematic project, do some tweaks and save with a new name. Potentially it could help, but usually it doesnt

What was the version before upgrading?

Did you try transferring the Project to the computer and opening it with the Maschine 2 (2.17 or 2.18) desktop application?