Closing on 5 unit property

[Jerry Johnson]

in the process of closing on a 5 unit. 4 of the units are occupied and have been for multiple years. The owner is a older gentleman and he took care of these tenants. I of the tenants be living there for over 12 years and another for 8 years.  So the rent is really low and should be up in the $700 +

how do i go about raising rent? some small upgrades are needed. The 5th unit need some TLC with great potential. 

just need some guidance on how and when to raise the rent step by step so I'm exceeding the process of getting good cash flow long term. I figure this process will take some time.. 

Please advise, Thank you.