This is a new major release. Please read the upgrade instructions carefully before you do the upgrade.
WHMCS V5.3.3 release Notes: http://docs.whmcs.com/Version_5.3_Release_Notes#Version_5.3.3
WHMCS v5.3.3 forum announcement: http://forum.whmcs.com/showthread.php?85050-WHMCS-V5-3-3-Released
WHMCS v5.3.3 CHANGELOG: http://docs.whmcs.com/Changelog:WHMCS_V5.3
WHMCS installation instructions: http://docs.whmcs.com/Installing_WHMCS
WHMCS upgrade instuctions: http://docs.whmcs.com/Upgrading
Friday, February 7, 2014