According to Einstein's E=mc2
With an atomic bomb, as it explodes the energy within the mass is being spent (becoming less).
Therefore the equation states that as the speed of light remains constant, the mass must therefore become less.
But mass cannot become less as it cannot be created nor destroyed.
Do you see the contradiction ?
There is no contradiction. You are making the invalid assumption that mass is conserved! In fact, mass is NOT conserved. The whole point of E=mc^2 is that matter can be converted into energy and energy can be converted into matter. You probably learned that mass is conserved in chemistry, but that is only so because in a typical chemistry lab the available measuring equipment is not capable of measuring the mass lost in an exothermic reaction or the mass gained in an endothermic reaction.