How do I raise rent on pre-existing & long-term tenants?

[Hayley Bartlett]

I'm looking into purchasing a 6 family apartment in Providence. RI.  The potential for cash flow is high, but the rent is currently well below market value by almost $300 per unit.  All units are currently rented out (2 bed/ 1 bath).  Additionally, most of the tenants have been long-term residents.  What would be the best way to go about raising rent?