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.
