Update existing Notion pages with a form
Make a form that edits existing pages from Notion, and pre-fills fields with data from the page.
1. Switch your form to update
mode
First, choose Notion on the "Integrations" tab:

Then specify that you want to
update
the existing page.
2. Use a formula to specify the page to update
You'll now get a formula that defines a unique link for every page in your table.
Create a new
formula
property in Notion and copy the formula Fillout provides on the integration page.
Note: adding a formula property is mostly for your convenience. If you already know the page ID of the page that you want to update, you can pass it into the URL yourself!
Note: if you don’t provide an “id” in the unique URL, the form will create a new page instead.
3. That’s it! The result:
Try out your form in preview mode, or click one of your unique links to update a page after you’ve published your form!
Your form is
automatically pre-filled
for whichever page it is updating. 
Note: in Notion, the formulas with the links to update the records are most easily clickable when using a Table view.