Unfortunately, you cannot directly update your KYC on the PNB FASTag website.
you have to go for IHMCL FASTag portal:

• Visit https://fastag.ihmcl.com/
• Log in using a mobile number.
  Click on “My Profile” and then “KYC”.
• Check your KYC status. If it’s incomplete, proceed to update it.
• Choose your “Customer Type” and fill the fields with updated ID and address proof documents.
• Submit the documents.