I found that in our demo Microsoft 365 tenant, there were Nintex updates for Nintex Form and Nintex Workflow as shown:

Nintex Workflow for Office 365 (Version: 1.0.4.0)




Nintex Form for Office 365 (Version: 1.2.4.1)  



But I got some hiccups while updating Nintex Apps for Office 365. So, let me give a step by step guide to update Nintex Apps.    

Step 1 

Issue: Nintex Forms for Office 365 within SharePoint Online, there is no option update from SharePoint Online Store

Follow this URL (Dated: Aug 8, 2017)

Step 2:

Sign in your Office 365 credentials and Nintex Form will be updated.  

Step 3:

Even after Nintex Form App update, it will still show, “An update for this app is available”.

Next, click on Nintex Form App. 

Step 4:

This time around you will able to get the button, GET IT as shown:



Follow this URL  (Dated:  29, Feb 2016)
https://community.nintex.com/docs/DOC-3493    

Step 5:
In few minutes, Nintex Forms and Nintex Workflow Apps will be updated.


 

The steps may wary for different Office 365 tenants a bit, this is overall process may help you.