How to update CRM record from fillable template

How to update CRM record from fillable template

Hi community,

Context: I have a pdf fillable document with 300+ fields in Zoho Writer. I've recreated these fields in the Deals module of Zoho CRM.

Complexity: Some fields are mapped and prefilled from the Deals module, while fields others are captured and filled on the fillable form for each deal by multiple parties.

**I am aware that a CRM record can be created from the fillable form once the form is submitted by triggering a function.

Ask:
Instead of creating a new record in CRM I would like to update the fields of an existing Deal. 
What would the basic deluge script look like to get the deal and update the fields in the CRM from the fillable fields.