Raising the rent on my tenants

[Jordan Akins]

My dad has around 33 units in a certain location here where we live in Ky. He has only raised the rate by $25 in the 25 years that he has had them. They seem to always stay rented with no problems at all due to the price, location, and them being pretty nice units. What is the best way to go about raising the rate to create more cash flow without loosing tenants?