Apple has apparently forgotten to renew its SSL certificate, resulting in certificate errors while updating OS X from the Mac App Store.
Here’s what the error says:
An Error has occurred
The certificate for this server is invalid. You might be connecting to a server that is pretending to be “swscan.apple.com” which could put you confidential information at risk.
Apple says that “swscan.apple.com” is one of the many servers that the company uses to deliver software updates. This server’s SSL certificate was valid from May 22, 2012 to May 24, 2014, and Apple hasn’t renewed it for validity after the 24th, resulting in all these errors.
If you’re facing the same issue, you can either directly download the OS X 10.9.3 update from Apple’s website, or change your date to May 22nd or earlier and try to reinstall it from the Mac App Store.