Check out Gareth’s tutorial on using Fillout to update existing Airtable records.
https://www.youtube.com/watch?v=mvDQBif-sbE&ab_channel=GAPConsulting
First, choose Airtable on the Integrate tab.

Then specify that you want to Update record.

You'll now get a formula that defines a unique link for every record in your table.
Create a new formula ****field and copy the formula Fillout provides on the Integrate page.

That's it! Finish setup. Try out your form in Preview mode, or click one of your unique links to update a record after you’ve published your form!
Your form is automatically pre-filled for whichever record it is updating. This can be changed in the top left area.