Go to "my account" by clicking on the arrow next to your name at the top right of any page. Click on "Account settings". Click on "Edit" next to your email address.
I thought it got fully implemented already. Seems not. Once done, that SHOULD be as complicated as it gets. For now the message you see is this:
Password
The change password feature is currently being worked on and is unavailable. As a workaround, if you need to change your password right now, sign out and use the "Forgot Password" option of the sign-in page.