To install OsCommerce manually on your e-Commerce hosting account, you should first of all download the OsCommerce files archive on your hard drive.
Here you can Download the osCommerce files
Once you have downloaded the release you would like to use, you can upload it to your hosting account using an FTP client or the File Manager tool under your Cpanel.
As soon as the upload of the file is completed, you can unpack it using the File Manager tool. In order to do that, you should access the file manager from your CPanel, find the oscommerce package, click on it and extract it using the "Extract File Contents" button