How could I downgrade python 3.7 to 3.6 which bring with mate 19.04?

Recently I find out that python 3.7 is annoying, as many libs are not fully compatible with it.
And one more thing, 3.7 see "await", "async" as key words, while many libs use these two words as function name , big problems happen !
I know I can't just run 'sudo apt purge python3.7', which will remove many essential libs the system need, but how could I downgrade python 3.7 to 3.6 ? Looking forward to your help.

IMHO the most reliable solution is to compile Python 3.6 from source so that it would be installed side by side with OS supported version in order to avoid incompatibilities.
Hopefully, the following thread might help

Thanks, I see it, but by doing this I will face great risk of destorying my system, maybe this mission is unpossible :joy:

Yes, replacement of the default 3.7 is not affordable, as stated by Peter Green. At the same time the next comment by Roman Lustrik cited by you as well, gives a hint how to avoid that.
Probably you might want to install virtual machine and try his recipe safely.