In WHMCS 7 you can use automatic update but in version 6 you till have to do this manually, including v6.x to  v7.x , which is shown below:

• Backup
• Rename live folder with pre-fix old-
• Extract the new package on the desktop
• Rename the admin directory on the PC version
• Copy the configuration.php from the live version to the desktop version
• Upload the desktop version to the /hosting/ folder on the live site
• Login and run/trigger the upgrade
• Delete the install directory
• Check the CRONs are setup properly
• Configure the update feature (Utilities/Update WHMCS/Configure Update Settings)
• Select an update channel and set a temporary directory for the update (/tmp)