There is a launcher used for starting apps and accessing settings on nexus 7 Android OS. It is much like a desktop on your computer, but you are able to change already-assigned default launcher to a new one.
How to Default a Launcher in nexus 7
* Unlock your screen and Touch Google Play in application drawer.
* In the store, press the "Search" button for a launcher replacement. There are some popular alternatives, like "Launcher Pro", "ADW Launcher" or "Zeam". You can try one of them and install it on your tablet.
* Go to "Settings" "Apps", tap your current default launcher and tap "Clear default". Then open the launcher you want to use and set it as default again.