Current Rent is $600 below market

[Jonathan Giglio]

Hello, I am considering buying a property that has a 6% cap rate and only cash flows about $200/month. However, one of the units is currently renting for $875/month. Using rentometer.com, I've determined the average 1bd/1br rent in this area to be $1,400. 

Does anyone have any stories about what they did in this situation? There must be legal limits as to how much you can raise rent. $500 a month in crease is a lot for me as an investor, but is also a lot for the tenant and likely unreasonable. The tenant has been there 15 months and wishes to stay.

What would you do? Thanks.