How to increase tenants rent?

[Allen Humphries]

So I just submitted an offer on my first deal. It is a 2800 square foot duplex 6 bed 4 bath (1400 sf on each side)... in a great location. Both sides are currently rented, but the offer is contingent on side B vacating upon closing because I’m planning on living in side B and renting side A. Both sides were severely underpaying for rent. Side A is paying $850 and staying upon closing, side B is paying $650 and leaving upon closing. The number I had in mind for these properties was $1200 a month and checked comparable rentals near by and asked for others opinions on what it should rent for and we all got around the same number... $1,200 a month. How do I go about increasing $850 to $1,200? Overtime, slowly? All at once and risk losing a good paying tenant that has been there for years and plans to be there for at least a couple more? Please help! And feel free to ask questions to clear anything up!