I am running OS 10.9.5 on an iMac and cannot install Seurity Update 2015-006 from the App store even after numerous tries.

I am running OS 10.9.5 on an iMac and cannot install Security Update 2015-006 from the App store even after numerous tries. I keep receiving the message "Security Update 2015-006 could not be verified. It may have been corrupted or tampered with during downloading. Click Update to download and check the update again.

Suggestions??? Thanks, Bob.