Solve W: GPG error in Ubuntu 13.04 / 12.10 / 12.04

Solve W: GPG error in Ubuntu 13.04 / 12.10 / 12.04

In Ubuntu 13.04 / 12.10 / 12.04, while updating your system you might face this GPG problems. “W: GPG error: http://archive.ubuntu.com raring Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key...
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