You can roll back to the 2.9.x code base by going to Ninja Forms > Settings > Advanced Settings > Rollback
When you click “Downgrade”, a popup will appear where you will need to confirm the downgrade.
When you rollback, your system will be returned to the point at which you upgraded to 3 and you will lose any changes that you made in 3.
Once you have rolled back, you can convert to the 3 codebase by following this documentation.
If you have any issues with the conversion, you can contact the Ninja Forms support team: https://ninjaforms.com/contact/