The first beta was released on the 6th of June, the second beta was released on the 24th of June, the third beta was released on the 11th of July, the fourth beta was released on the 22nd of July and the fifth beta was released on the 6th of August.
Apple also released iTunes 10.5 Beta 6 to developers. The new version of iTunes is required to sync iOS 5 Beta 6 devices.
Once you have the latest version of iTunes, please note that you wont be able to update devices running older beta version of iOS 5.