The only way to "uninstall" Windows 8 (revert to an earlier version of Windows) would be to do a fresh installation of Windows 7. You should have a Windows 7 product key on the laptop. SInce you don't have a DVD drive you can create a bootable USB flash drive
That will result in a plain installation of Windows: you'll need to update your drivers from dell.com and re-install any software and games. Also, it will delete any information on the drive so you'll need to make sure any documents, pictures, etc are backed up.