The automatic updater tool is located under the Utilities menu within the WHMCS admin area in version 7.0 and above. It is accessed via Utilities > Update WHMCS.
If you are currently using an older version the automatic updater is not available. Please upgrade manually to the latest full release version in order to take advantage of this feature.
The latest full release version should be downloaded from http://download.whmcs.com/Manual upgrade instructions are located at http://docs.whmcs.com/Upgrading#For_a_Full_Release_Version
If you are already running version 7.0 and above but do not see the option, it means your administrator role does not have permission to access the page. To grant this permission, navigate to Setup > Staff Management > Administrator Roles, edit your role and tick the Update WHMCS permission