Welcome to the Airtable community!

An Airtable form always creates a new record.

Here are some options for you:

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  Have a form for uploading the file, and also include something that uniquely identifies the original record. You can prefill the uniquely identifying information in a url button field. (My app Ready-Made Formulas makes building the formula for the url very easy, and this formula is free.) Then have a scripting automation that moves the uploaded file to the original record. I have written several versions of scripts that do this for clients. (The script can replace any existing attachment, append or prepend the new attachment to existing attachments, or move existing attachments somewhere else.) This method also protects the privacy of other data in your base, can provide error handling if the user messes with the prefill, and can delete/clean-up the no-longer-necessary record that was created by the form.

  
  


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  Have a form for uploading the file, prefilled with uniquely identifying info as before, but use the find records action to find the original record and update record action to move the attachment, or to create a link between the original record and the new record with the attachment. This method is not as robust as the scripting method, and cannot delete the no-longer necessary record that was created by the form, but can be made to work.

  
  


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  Use a 3rd party “portal” tool such as miniExtensions, Stacker, etc.

  
  

Thank you very much @kuovonne for the solution. I will try to implement the button with the scripting automation. Fantastic community