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 upload a new .htpasswd file. This will be at the top level of your site (above the www) and called .htpasswdosc, or it will be in the www/catalog/admin folder and called .htpasswd. You can determine which is the case by downloading via FTP and examining the contents of the file .htaccess in the www/catalog/admin folder.

To change your username or password, follow the instructions for setting up a .htpasswd file here, although taking into account the different location of your .htpasswd file.