I wouldn't call it "changing" your gender for starters. Isn't the point that you already are your gender and it's just not the one you were assigned at birth? So, I wouldn't say you want to change your gender - it is already there. Present it as a fact and not like "I want to be someone else". The fact is: you already are a different gender than what they're calling you. You're not going to change and you haven't changed either. You've always been who you are now, you just hadn't realized it yet. It is important that you are still the exact same person as you were before - you just would be a lot happier if people accepted you the way you are and called you who you are. Your gender is not a choice - accepting it is. And you have decided to accept yourself and hopefully they can too.