You can change your password in your Profile area.
Click on the Profile menu button in the main menu bar at the top.
Click on the Modify Profile button toward the top left of the screen and from the drop down list select Account Settings.
You can now enter a new password on this page, don't forget to save it by clicking the Change Settings button at the lower right of the page.
You will need your old password to be able to do this, if you have forgotten your old password you will have to use the reset password facility instead. You will need to be logged out to do this.