That’s not how forms work. Forms will ALWAYS add a new record. If you want to update an existing record, then turn on editing for that existing record when viewing it’s details.
Otherwise you will have to do some sheet magic to find the last row in a form response sheet and update values on an existing row in a different sheet. Or you can use a relation and rollup to find the last record, then create another relation based on the rollup value and probably a template to get that last row. Then use lookups to get the the relevant data from the form response sheet.
I would instead just make sure they can see their existing reservations and allow them to update the record with the edit functionality.