We’ve had numerous requests for sample data for Magento’s latest release and are making it available. The archive file is available in .zip (40 MB), .tar.gz (40 MB), and .tar.bz2 (36MB) formats to download (the files contain exactly the same content inside and are provided in different formats just for your convenience).
Each archive contains a SQL script that should be applied on a clean database with your database management tool (phpMyAdmin or whatever you use), and ‘media’ folder with sample product images that you should copy to your Magento media folder. You should then run the regular installation process as for a complete package. Please make sure that you have exactly the same version of Magento that the sample data is provided for – 0.7.14800.
Please also backup all your data and keep separately any custom changes on the files you did, because they may not be compatible with the sample data provided.