Firstly, you'll need to be using one of the paid versions of SugarCRM, Professional, Enterprise, or Ultimate editions. Secondly, you'll need to upgrade from 6.5.2 to 6.5.18, .19, or .20. Once that's completed, please use the upgrade documentation to complete the upgrade process from 6.5.18 (19 or 20) to 7.5.1.0
If you are using Sugar Community Edition, we do not offer an upgrade to Sugar 7 at this time.