Form submissions will always create a new record, so the best you can do is delete the newly created record after you've used the data in it.
Do you have a Pro plan? If so, you can add a "Run a script" action and write some JavaScript that will delete the form submission once you have utilized the data in the form submission to update the existing record. There are some examples of the code that can accomplish this in the forums and you can search for it
If you do not have a Pro plan then you could attempt to use a third party tool with some JavaScript to delete the record the form submission created
