How do I apply Title Case to just a portion of a text field

I have an address field in all caps and I want to convert just the city to title case. So I currently have:

AKRON, OH 12345
BREA, CA 54321

And I want to see:

Akron, OH 12345
Brea, CA 54321

Basically I want to know how to convert just a portion of a string, in this case the portion before the comma.