Error occured while updating the System

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.

Affordable-Laptop 27-11-18, 9:05 p.m. JitendraBafna

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.

27-11-18, 9:46 p.m. Srikant

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