Downgrade K2 Five 5.9 to K2 Five 5.4 Due to Incorrect Upgrade Path

Hi, anyone know how to downgrade K2 five 5.9 to K2 five 5.6?

Before this, the old version is K2 Blackpearl 4.7. Now I'm upgrading to K2 Five 5.9 version and I have countered issues where some forms and views cannot open. After doing some research, I just know that it cannot install directly to K2 Five 5.9. 

Below is from Nintex:
“Upgrading Nintex K2 blackpearl (4.7) or Nintex K2 Five versions (5.0), (5.1), (5.2), (5.3), (5.4), and (5.5) to latest version of Nintex Automation is not supported.  You must upgrade from a previous version to at least Nintex K2 Five (5.6) or Nintex Automation (5.7) before upgrading to the latest version of Nintex Automation.”

So how can I solve this problem? Should I downgrade the version or is there any solution to resolve this issue? If I need to downgrade the version, what is the proper way to downgrade the version? Should I just uninstall the version and go back to version K2 Five 5.6 or K2 Blackpearl?