The first step is to make sure that your Ninja Forms plugins are up to date. Make sure to disable any non-official Ninja Forms plugins.
If you are eligible to upgrade to Ninja Forms 3.0, you can go to Forms > Settings > General Tab > Advanced Settings and you will see a button that says “Update & Convert Forms”
On the Conversion page you will see a list of your forms that are able to be converted to work with 3.0
If you have any issues with the conversion, you can Roll Back to the 2.9 codebase.