I want to change the spelling of my first name on my birth certificate. It is now Ricki
And I want to change it to Rikki. How do I go about that?

You don't change your birth certificate. You go to your local courts, possibly probate court, and petition for a legal name change.
If you are a minor, your parents will have to do it. Done all the time, your petition will be publicly posted in your local newspaper for 3-4 weeks.