Buying Rental Properties With Below Market Tenants

[James Lucero]

Hi all, 
I'm looking at some single family properties in the area and there is one for sale that says the current tenants are paying $500 below market value. I've read a lot if posts about the difficulty of vacating a tenant, how would one go about raising the rent for said tenant? (House price reflects value with the extra $500 that are currently not being charged.) TIA