You can easily change your administrator’s password from the Joomla admin area > Site -> User Manager. Once there, check the box next to the administrator username and click the Edit button located at the top-right Joomla menu.
In the Edit menu you should type the new admin password in the New Password and Verify Password fields. When you are done with the change, click the Save button.
You should use the new admin password next time you login to your Joomla admin area