App Upgrade
If you have installted the app before 3rd of May 2020, you will be promted by Shopify to update the app when you access the app, the reason for the required update is that the newer version of the app requires access to read and update your theme in-order to insert snippets for newer features of app like Collection Add-ons.
No Prompt to Update the App
If you are already logged into Shopify within a browser session which has been open for a long period then you may not get prompted to update and accessing Collection Add-ons tab will show an "Upgrade Required" message. In that case you and try one of the following two options:
Loggout of Shopify and close all your browser windows, reglogin and try and access the app, this time you should be prompted to updated, if you are still not promoted to update the app, you would need to manually remove the cookies for https://product-addon-admin.herokuapp.com/ domain and re-try to access the app, below. Gudie to Removing cookies on Chrome
Loggin to shopify using Incognito mode of your browser and access the app you should be promted to update the app.