To change your OpenCart admin password, log in your OpenCart admin panel and go to
System » Users » Users from the admin navigation menu.
Click on "Edit" opposite to the admin user.
Then simply type and retype the password in the "Password:" and "Confirm:" text boxes. Note that this page allows you to change the username of the administrator user and its contact details. You can also assign multiple administrators by selecting "Top Administrator" from the "User Group" field.
Once you've entered the new password click "Save" in the upper right corner to apply the changes to your admin user.
Now your admin user password has been successfully changed.