By default, SSLVPN service uses self signed certificate. This article describes about generating new CA signed certificate and using it on SSLVPN service.
This release includes significant user interface changes and many new features that are different from the SonicOS 6.5 and earlier firmware. The below resolution is for customers using SonicOS 7.X firmware.
Navigate to Network | SSLVPN | Server settings | Under SSLVPN server settings drop down certificate selection and select the imported certificate. 
This release includes significant user interface changes and many new features that are different from the SonicOS 6.2 and earlier firmware. The below resolution is for customers using SonicOS 6.5 firmware.
Navigate to Manage | Connectivity | SSL VPN | Server Settings drop down certificate selection  and select the imported certificate.