In order to change the password of an Administrator user you need to be logged in as that particular user.
Click on the Preferences tab in the Admin of your website
Click on the Website Administrators tab
This will take you to the Administrator accounts page where all website administrator accounts are listed.
You will note that there is a "Change Password" link only next to the Administrator that you are currently logged in as.
Click on the Change Password link - this will take you to the Change Password screen
First of all type in your old password
Then type in your new password
Then verify your new password
If you are unable to think of a new password you are able to use the randomly generated passwords listed at the bottom of the page.
When you have set your new password click on the "Change" button. this will save your changes and take you back to the Administrator Accounts page.