To change your password, go to your profile drop-down in the top right of your screen (1) and select 'Change password' (2).
In the pop-up, enter you current password (1), followed by the new password you wish to change it to (2). Confirm the new password (3), and click on 'Change password' (4). You will be warned if your current password is incorrect, or if your new password and confirmatory password do not match.
If you managed to successfully update your password, your old password will no longer be accepted the next time you log in, and you will have to use your new password.
Forgot your password? See here for instructions to reset it.