How to raise rents to tenant in multifamily?

[Imelda Mendoza]

We purchased a 5 multifamily in Corsicana, TX last year and have slowly updated a few of the units that have became vacant.  New rents for update ones are $775-$850.  We currently have 2 units that have not been updated and rents are at $550 & $770 both have been living there 3yrs+ with zero rate increase in the past 3 years.  

Do you mail out a letter? If so, any templates you recommend or do you call them?

Thanks in advance!