I believe you have two options. Reload a previous save or blow up Megaton. I recommend the first option, because keeping Megaton intact is far more beneficial than blowing it up. If you do not have a previous save, go find Mister Burke inside Moriarty's Saloon in Megaton. He will task you with detonating the bomb. Once you complete the task you will get the achievement. Note: it is considered a very evil act.
Before blowing up Megaton, make sure you:
-get the Strength Bobblehead from Lucas Simms house located upstairs on the desk
-start The Wasteland Survival Guide quest by going to Craterside Supply and talking to Moira Brown