For an NRI to update FATCA, they need to contact their respective Dealer/Relationship Manager or Customer Service at service.securities@kotak.com.
The respective RM / Dealer / CS Executive will send a KYC updation form (which contains the FATCA declaration) to the NRI client which the client needs to fill & attest the relevant documents and courier it Kotak HO.
It is a chiefly an offline process for NRIs and cannot be done on the KST App or Kotak Securities Website
Or download the form from here, fill and send the scanned copy to service.securities@kotak.com