When you see the Upgrade Connection button beside an app in the Connected Apps page, then you have probably previously connected your Airbnb account to that app directly, before connecting it to Hospitable.
At the current state, your Airbnb listings have a duplicate connection - one to Hospitable, and one directly to the app. This duplicate connection has no effect on how your listings are being managed in Hospitable. The features are working as normal. This simply means that the connected app is currently able to sync with your Airbnb listings only.
Why do I need to upgrade?
The app will be able to sync to your listings on all platforms, not just Airbnb.
The app's API access is expanded which means they will have the possibility to:
Automatically tag your properties.
Create Direct bookings in the custom sites you have created with them.
Update your calendar's price, availability and min stay (as long as you are using Hospitable's upgraded PMS connection with Airbnb).
Respond to reviews.
... and more.
Note: This does not mean that the app will be doing these upon upgrading. It just means that if they develop these features in their app, they will be able to support that in their integration with Hospitable.
What happens when I click on Upgrade Connection?
You will be redirected to the app's website. The next steps from there will vary depending on whether the application requires additional configuration related to upgraded integration.