From the Symbian Developers blog:
The new “Chat history” settings tab allows you to Send, Backup or Clear chats manually. Earlier, WhatsApp used to back-up the conversations automatically and Sharing was already there. Clear all conversations option was added in last build which also takes place as a separate submenu Chat history tab.
The update can be downloaded from the main WhatsApp site or the SIS installer can be directly downloaded here. If you're already a WhatsApp user then the upgrade is a no brainer. I've used it in the past but because I swap phones a fair bit, the app's insistence on auto-starting and hooking onto a particular SIM card did tend to get in the way, which is why I've linked across to Symbian Developer's piece and screenshots here.
Comments welcome if you have something to say on the new functions.