If you are already signed in and would like to change your password, click on your name in the sidebar and select "Your Account".
Select the Security tab at the top.
Click the "Change Password" button and you will open a left hand menu with the following fields:
Current Password: you need to enter your current password that is set for your account
New Password: enter the new password you'd like to set for your account. Refer to Password Requirements for rules on setting passwords.
Password Confirmation: to make sure you've typed your new password correctly, you need to re-enter it here
Simply complete these fields and click "Save" in the top right to save your changes.