If you have forgotten your password for your account. You can request a reset by selecting "Forgotten password?" link provided under the login box. You will then be asked to enter the email address that you signed up with. Once complete an email will be sent to that email with a link. Once you click on the link. You can then update your accounts password.
If you know your password, but simply want to change it. This can be done via your profile page. You can get there by clicking on your user in the top right.
Then selecting profile and selecting the "User profile" tab.