Are you a Site Collection Administrator on the site where you are trying to update the app? For any installation or changes to the app, it requires SCA permissions.

https://help.nintex.com/en-US/o365/#o365/O365WorkFlow/Admin/Minimumpermissionsworkflow.htm

Yeah Burked,  I Site collection admin as well as SharePoint admin

@svignesh If you are a SCA and you are unable to update the app, I would recommend opening a case with nintex support using the customer central link in the left hand bar.

@svignesh can you try update the app from Site Contents?

Failing that please remove and then re-add the app as per the steps below:

- Remove the Nintex workflow app 
- Once the app is removed, re-add the app to the site. 
- When the app is adding you may see two "ghost" versions of the app, one being added and the other being removed. 
- Cancel the app that is being added to the site, and wait for the "ghost" version of the app to fully remove itself. 
- Once the app is gone from Site Contents, add the app to the site one more time. 
- You will only see one copy of the app within Site Contents (the version being added), and once it has finished adding, the app icon will now be available within the list ribbon. 

@svignesh , did you have an answer to your question? If we remove the the Nintex forms and workflows apps. will that remove all the existing forms and workflows?