How to raise rents on new 4-plex?

[Eric Denson]

I am closing on my first investment property within the next week. Yeah!!! It's a 4-plex with tenants. The rents are $350 for a two bedroom one bath unit where the owner pays the trash and water. I've called two other local small apartments like mine and the rent is $400 for the same thing and no vacancies. What's the best way to get my rents up to market? Instantly or gradually? I'm not interested in major tenant turnover right up front so I was thinking re-new everyone's lease at $375, make the improvements I want to make and then all new leases would be $400. Comments?
First post BTW. Hello everyone!!