How to change realm?

I quit WoW for a long time. Just yesterday, my friend said "come play at Warmane, I am in "icecrown" realm. So, I started opening the account and join the game. When I log in the game, it says "Frostmourne" realm. How do I change it so I can play with him?

On top of the character selection screen is "change realm". Click that and choose Icecrown.