When I am trying to update the system, It gives me following error.
W: GPG error: https://ld.iitb.ac.in/debian stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 61FDCD06FA513CDC
E: The repository 'https://ld.iitb.ac.in/debian stable Release' is not signed.
Just recently I installed the new OS using ISO file.
PFA of error screenshot.
Please let me know how can I resolve that problem.
If you have reinstalled the OS, then I suggest you please run update-2-offline from this link.
The above update has many patches you probably need, including a fix for the public key issue.