To dismantle something means you take it apart in a process, piece by piece. To demolish something (demolition is the noun) means you destroy it. Less care and probably not much method.

A car at a junk yard is dismantled. A wrecking ball on a crane demolishes a building. Slang usage of the words: If I don't pay him back he is going to dismantle my face. I am so hungry I am going to demolish that pizza.