This release is a maintenance update and it addresses issues founded since the previous 7.6.0 one from July 2018. The easiest way to update is to use WHMCS built in Automatic Updater. In you WHMCS admin go to to Utilities > Update WHMCS to begin the process. If you do update for the first time, you need to set up a temp directory. Remember to backup database! Manual downloads are also available from WHMCS website, there you could find the full release and incremental versions. WHMCS v7.6.1 requires Ioncube Loader 10.1 or higher.