Is there a way to update existing records with forms ONLY using AirTable?

Solely using Airtable? No.
But the closest thing you can get is this.

No, there is no easy nor reliable way to do this in Airtable on its own. It requires creating a constantly-changing prefill link for your record (which must be changed every time your record’s data changes), and then you’d have to create an automation to merge the new form submission’s record with the old record, which is especially tricky with linked record fields and attachment fields.

All of this is extremely fragile, difficult to maintain, and requires using a constantly-updating URL.

Instead, your #1 best bet for updating records from a form is to use Fillout, which is easy, free, and lets you update records from a form.

- ScottWorld, Expert Airtable Consultant