Changing your password is a fairly simple process. Here are the steps to follow:
1. Click on your username in your account bar at the top right of the site.
2. Click on the edit tab.
3. The third and fourth text fields are titled "Password" and "Confirm Password". Enter the new password and retype it in the "Confirm Password" field.
4. Scroll down to the bottom of the page. Under the text field containing WSO's Terms & Conditions of use is a "Save" button. Click it.
Related: How do I change my username?