It's a bit long-winded. Click on your name in the top right, you get your profile page with your karma and a list of websites. One of them is Garage. Follow that. On that page's top right, you have two links, "Log Out" and "My Account". Follow "My Account" and there, somewhere is the middle of the right pane, is the "Change Password" link.

EDIT:
Ehm, I forgot one step. You have to leave the Wiki page You need to go to for example Community or Development first. What a wonderful feat of UI design, eh?