Thanks for your question! In the Survey123 app you can modify surveys you may have completed. For example, say you capture data with one survey and you keep that data in your Draft folder. You can go back at any point in time, open that Draft and modify any of the answers. You could in fact also edit surveys in your outbox as well. So say you are capturing data while disconnected and you submit the data. Since you are disconnected, the surveys will be kept in your outbox folder in the Survey123 app, waiting for you to send them once connectivity is back. While in the outbox, you can also open any survey and modify it.
Once a survey has been sent (will be moved to the sent folder in your Survey123 app) you can still open the survey while in your device but the modifications you make will be persisted in the feature service as a new record. You can think of this like email: you can edit your email messages while in draft mode and even while in your outbox... but once an email has been sent, you cannot edit it anymore since it is already in someone else's inbox folder. If you were to edit the email from your Sent folder, your modifications would be sent as a new e-mail, rather than overwriting your old e-mail.
What we mean we say that Survey123 cannot edit data is that you cannot get features that existing in a feature service and change them from the app. This is a known and temporary limitation. The intent is to implement editing capabilities in Survey123 so you can fetch features (surveys) from your feature service and then change them while in the survey123 app.
