This is a maintenance release for the 6.3 series of WHMCS that includes bug fixes & other updates.
Both a full release and incremental patch set upgrade are being made available. If you are running the immediate previous release 6.3.0, you can use the incremental patch set to upgrade.
Posted by Matt on Wednesday, April 13th, 2016
Read more @ http://blog.whmcs.com/?t=113291