If DNA methylation inactivates genes, does DNA demethylation activate them?
Here is a series of articles explaining DNA Demethylation from Sience Direct. 

In the most broadest of senses, DNA methylation adds methyl groups to DNA preventing the transcription complex from forming and transcribing the gene into mRNA. By demethylating the DNA, it allows the transcription complex to form and mRNA to be transcribed for translation.

When it comes to cancer, Most of the tumor suppressor genes are down regulated to allow for genes like stat3 to be upregulated. Ideally, by removing the suppression of these genes, they can reduce the levels of oncogenes and slow the growth of the cancer.

Think of it like this A -| B. Cancer decreases A so B can increase in expression A↓ -| B↑. By demethylating A, we  A↑ -| B↓.