Hover over your username in the top-right corner and click 'Profile'. Then select the 'Edit profile' link on the left side of the page. Click the 'Email and Password & Facebook Connect' heading to expand the appropriate section to edit your password.
Be sure to click the Save button at the bottom of the page to save your changes,and please note that logging out of your account will cause those changes will take effect immediately.
Please feel free to write back if you have any other questions.
-Kim, Community Manager