To change the password associated with your Coursio account, you must first be logged into the account. Then go to Settings in the main menu and then select Change Password. There you have to enter the new password you want and also the old password. Once you have selected your new password, go to the bottom of the page to click Save for your new password to take effect.
See below video for how to do it.
If you forgot your password and can't log in, you can read how to reset it here.