Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen atoms.In biochemical systems, the process of demethylation is catalyzed by demethylases. These enzymes oxidize N-methyl groups, which occur in histones and some forms of DNA. R2N-CH3 + O ? R2N-H + CH2O One such oxidative enzyme family is the cytochrome P450 Alpha-ketoglutarate-dependent nonheme enzymes are active for demethylation of DNA, operating by similar pathway.

Reference
Wiki: Demethylation