Taha,
When you download version 7.0, there is a file included in the download called DOCS. Inside this folder is the Implementation Guide. This guide walks you through upgrading.
Please keep a couple of things in mind, Version 7.0 does not support Exchange 2003 and the settings that are currently in 6.0 can not be imported into 7.0.
As Technical Support, we recommend uninstalling 6.0 and performing a new installation of 7.0 instead of upgrading over the 6.0 installation.
Good Luck.