If you follow the directions I linked above, it’s pretty easy to append rows.
If you must overwrite rows in the Sheet (rather than appending new rows and using another Sheet as a “rollup view” to show one row per Email Address) then you can’t use a simple form post. Instead, you’d need to call a webhook-compatible service that in turn calls the Google Sheets API to update cells.*
*Technically, every Google Form has an unguessable ‘Edit Link’ that lets a person go back and
edit their answers after each form post. Very handy when using a visible Google Form, but
when skipping the visible form to POST straight to the form endpoint, it’s not feasible to get
the Edit Link from the response. If you had the Edit Link you could POST updates to that but alas.