Looking to raise rent without losing tenants

[Karl Dugal]

Hello all! My name is Karl and I have been a landlord for 7 years. Up until now I have never raised rent on existing tenants, waiting until they move before increasing price . My wife and I bought a 2 family home this past summer (July 2016). We inherited tenants in 1 of the units and agreed to honor their lease (at considerably below market rent). The lease is up in 3 months and tenants already emailed to express their desire to stay for another 2 years at the current rent. While they have been ideal tenants, I absolutely need to raise rent- as I said, it is currently well below market value. My question is: what is the best way to raise rent to near market value without losing tenants? Right now I was thinking of raising it $120-$170/month for 24 month term. Thanks for your help and look forward to reading replies Karl