If you don't know what your password is, you cannot change it when you are logged in.
If you log out by mousing over your name on the top right hand part of the screen, a menu will drop down, and click Sign Out. That will sign you out. Then at the right top of the screen, it will say Sign In or Join Us. Click Sign In. That brings you to a log in screen. There is a "forgot my password" link right below the password box. You need to know the email that you used to join the board. (If you don't know that, before you sign out, mouse over your name and click Edit Profile. On the Personal tab of your profile, you'll see the email address associated with your account.