Have you seen Product Update - What you need to know?
Note: If the Status field says Service Unavailable, then IIS is restarting. Refresh the page until the solutions display.
If any errors appear for installed solutions, then the solution package upgrade failed. To resolve the error, click the solution and then click Deploy Solution. This should deploy the solution. If not, reset IIS (Start->Run->IISreset) before clicking Deploy Solution again. Resetting IIS resolves this issue as it releases all locks on the application files. After deploying manually, reset IIS on each front end server.
Note: If you receive an error when attempting to access Nintex Administration page, ensure that nintexcommon.wsp is deployed globally. If you cannot see Workflow environment or Forms Environment sections in Nintex Administration page, ensure that nintexworkflow2016.wsp, nintexworkflow2016core.wsp, nintexforms2016.wsp, and nintexforms2016core.wsp are deployed globally. To view deployment status, on the Central Administration Home page, click System Settings, then click Manage farm solutions in the Farm Management section. See Deploy the solutions in the installation guide for information on the deploying solutions. (Note: The deployment option “All content web applications” does not include the web application.) No IISrest is required for deployment to the Central Administration web application.
For instructions on accessing the Services snap-in, see the Microsoft article Services.
I thought I'd share my experience with upgrading (from 22.214.171.124 to 126.96.36.199) and, in particular, the issues I encountered and how I went about resolving them.
Firstly, at step 3 (above), the Solutions Management page was reporting the nintexforms2016.wsp and nintexforms2016core.wsp solutions as Error. The suggested fix is to select the solution, then click the Deploy Solution. However, when I went into my failed solutions I had no such option. Instead, I could Retract Solution -- which I did -- and once the retraction completes I was able to Deploy Solution. Once the deployment completed, both solutions were successfully reporting as Deployed.
However, I still had issues as no Nintex forms were working. For example, when attempting to edit/design a form, I was getting the following error:
The Expression prefix 'NFResources' was not recognized. Please correct the prefix or register the prefix in the <expressionBuilders> section of configuration.
To resolve this, I needed to re-activate the Nintex Forms feature on the web application(s) as suggested by this blog post. I don't know if this is normal practice for an upgrade (i.e. the feature gets deactivated) or a consequence of the error I experienced. Hopefully this post might help someone in future, and/or perhaps this document could be updated with the additional information on troubleshooting the error, if deemed appropriate.