Unroot LG G4

I bought a used T Mobile LG G4 off of ebay. When going through the settings i saw that the previous owner had flashed a different rom to the phone. Currently the phone says H815 which is the international version of the LG G4. I was hoping that when LG released Marshmallow it would just wipe away to flash but it seems as though I can't update with the phone as is. Does anyone know a simple way to unroot/unflash the software? TIA