Is histone methylation reversible?

Yes, histone methylation is reversible. Histone methylation is an epigenetic process that alters the physical structure of DNA but does not change the DNA sequence itself. 

Histone methylation was earlier thought to be an irreversible process. A group of scientists at Harvard Medical School accidently discovered histone demethylase, which is the reverse of histone methylation, in 2004. They found that a newly discovered protein repressed gene expression by demethylating a specific histone linked to active transcription.