Microsoft isn't saying anything about what this Windows RT update will include, aside from the fact there will be an update. The company already has stated that Windows RT devices will not be updated to Windows 10, so this new update will likely involve just minor changes to ensure the OS remains usable and compatible.
This update that Microsoft will be delivering to Windows RT devices could be the last update they ever receive. It has been clear for a long time that Windows RT is a dead operating system with no future, as Microsoft looks to consolidate their range of operating systems into one version for x86 devices (Windows 10) and one for ARM devices (Windows 10 Mobile).
Even when Windows RT existed as a platform for ARM-based tablets, it was never successful. Currently there are only seven Windows RT devices in the wild, and none of them (including the Microsoft Surface RT, Surface 2 and the Nokia Lumia 2520) sold in any significant quantity. All Windows RT devices are now discontinued.
With Windows 10 set to launch on July 29th, it shouldn't be long before Microsoft reveals what this Windows RT update involves.