You can manage the assignment of a domain with a given service from the 2FAS app level. Tap edit on the tokens list, choose the token you want to edit, and tap the “Browser Extension” option. There you will see a list of domains with which the service/token is paired. You can delete individual assignments, but you can’t modify them.
To add an assignment, send a request for a token to the mobile app from the browser extension level. Then in the app, choose the correct service/token with which the domain is to be associated.