How to go back to V20 Nexus ?

Hi there,

since the automatic update to Omega Kodi is no longer working on my 50” Android TV.

It still has my library’s and it still can access my NAS-Systems (which are in another building, in another network).. 
But I can only play 1 File (one Episode, one Movie or one Song. After the file was played Kodi is completely freezing or telling me that it can’t find any of my network devices.

Same with scraping new Movies,Series or Music. Kodi starts and then freezes at some point without updating the library’s.

As I was fully satisfied with v20 Nexus I would like to revert back to it. 

How can I install/revert back to the older version on my Android TV ?
How can I then stop Kodi from updating automatically to Omega again ?

Help for a dummie user would be appreciated,