osCommerce uses .htaccess and .htpasswd format files to manage the protection on the admin section of your shop.

If you need to change the password or username associated with the admin section, you need to create a new .htpasswd entry. This is an encrypted password oscommerce uses to protect the admin area. First browse to where the .htpasswd file is located through the cpanel's file manager.
Click on file manager in the control panel.
You should be in the home directory click on the .htpasswds folder icon
click on the public_html folder and navigate to the oscommerce installation folder, then to the admin folder.
You will see a file passed click on the name of this file, and select edit in the top right hand side menu.

Now go to our page for creating an .htpasswd here this will help you create .htpasswd. Copy the result text and paste it into the file you are editing from the file manager.

Then hit save at the bottom of the edit page form.

You should now be able to log into the oscommerce admin section with the new username and password.