New owner seeks advice on how to raise the rent

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We are out of state investors that just bought a triplex in Mesa, AZ. All three units are rented out to reliable long term renters (minimum 2 years) but are about $150 under rental market value. All the units are clean and do not need renovation (done in 2014). I am seeking advice on strategies to raise the rent but keep the current tenants. Thanks for your help!