I know with Hosted Feature Services from ArcGIS Online, that you have to Right-Click on the Hosted Feature Service name, then go to Edit Features, then click on "create Local Copy for Editing". This will create a local copy on your computer that will allow you to start editing the features.
When you complete your edits, then you will have to right-click on the hosted feature service name again and then select "Synchronize edits to local server" and this should push your edits back into the hosted feature service that Survey123 created.
HOWEVER, I STRONGLY recommend trying this on a test data set first to ensure there are no issues with this workflow; as I have NOT tried this with a Survey123 generated HFS.
Alternatively, you could use ArcGIS PRO to make the edits to the Hosted Feature Service through direct connection. You have to be careful as you are working directly with the "Live" data set.
Good Luck!