HELP!!! MEDICAL CARDS UK! NHS?

[Aran]

FIRST I DO NOT NOW WHAT CATEGORIZE THIS QUESTION UNDER! Anyway, how long does it take to change your address on medical cards? I still get letters from my gp, but on the actual card, it says me first address. Can someone tell me how long does it take to update it and how much does it cost? If it helps i am Under 16,

UK ONLY!