Hope this helps:
Requisites:
CoreELEC installed to eMMC in dual boot mode.
Procedure:
- Boot CoreELEC from same media (SD or USB) used for dual boot installation.
- From shell, execute ceemmc -x
After completion the box should boot to Android again.
Warning:
If single boot mode installation was used, the Android firmware has to be flashed back to the box.
More details can be found HERE.