Here are the steps:

1. Navigate to your Azure DevOps  page

2. In the top left hand corner of your Azure DevOps page, click your profile picture. A dropdown box will appear. Click on My profile .

3. In the left panel in the last row, click on Manage Authorizations.

4. A pop up box will appear. Scroll to App Center and click on Revoke in the far right hand corner. 

5. A pop up box titled Revoke Access will appear. Click on the Revoke button again.

You have now revoked access for the App Center application from Azure DevOps.