OK, I just checked the system specs for your model, and you should able to upgrade just fine to Windows 7 itself. Once you have upgraded to Windows 7, you will be automatically prompted by Windows to carry out a FREE upgrade to Windows 10 (that's the latest version of Windows available as of yet).
You may purchase a DVD copy of Windows 7 online from Amazon, using the following link:
Once you have gotten the installation disk, just power on your system with the disk inserted and follow the on-screen directions to boot up from the disk, and install Windows 7 accordingly.
One more thing: your Dell system has 2GB of RAM installed; while that's the minimum requirement to install Windows 7, I'd advise you to upgrade your RAM to atleast 4GB, so as to ensure smooth performance with Windows 7 (as well as Windows 10.)
You may purchase the additional 2GB of RAM online as well, using the following link:
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