I assume you are using a form or a form container for the welcome screen? I would guess a form container based on your description. Forms will always add a new row when submitted.
Instead, you have a couple of options. First of all, I would make sure the tab screen is linked to the user table and filtered to the signed in user row. From there, you could add a button that opens a separate edit screen, but I think the better option is to place entry components directly on the tab detail screen. Essentially avoid using a form or form container and directly update the row in real time. Since the screen would be filtered, it would update the user’s row.