How do I right a letter to tell my client that I am increasing there maintenance fee?

Hi! kimah, welcome to AMHD :)

No one likes to receive a price increase letter, but at the same time you shouldn't be expected to furnish a service at a loss.

Be as short as possible, but indicate how costs have increased for you; taxes, fuel, lawn treatment supplies, maintenance on your equipment, storage, salaries, etc..

If you've kept records you could indicate the difference from past years to this year, and then to your projected costs for next year.

You could include a coupon for future services, and leave a way you and your customer can communicate to formalize future services.

See here for more ideas:
AMHD - Price increase letter threads