In order to change your password you need to be logged in to your account. (If you have forgotten your password click on the 'Forgot Password' link on the login page)
Start by navigating into your profile page by clicking on your picture and/or name in the top of the navigation pane in the left hand side panel.
Once in you will be able to view your details. Click the edit button in the top right corner. Here you will be able to edit your details and further down towards the bottom of the screen you will be able to enter your new password. Don't forget to confirm your new password in the second password field and click the save button.
Thats it.. you're all done! Your password is now changed.