I don't think it has UEFI. You can install 7 on it with the product key providing that product key is meant for 7. Keep in mind it has to be for that specific version. So if the product key is for Win 7 professional, then you have to use professional and not ultimate as an example. There's little diffrence between ultimate and pro anyways. Unless you need Bitlocker which I would avoid like the plague and use something like Veracrypt. Just sayn'.
If you want to install 10, you'll need to buy 10. UEFI or not. You can obtain the Dell install media on the net or they sell the CD on eBay. Remember, it has to be the same version as the product key.
To install the OS. While the computer boots keep pressing the F12 key. You should have the option to boot from CD. Let it install and do not boot from CD once it reboots and asks. Just let it do its thing. You could also press F2 and change the boot order in BIOS. F2 will take you to BIOS. If you install or have an SSD, make sure that AHCI is enabled in BIOS. Do that before installing Windows and if you are using a SSD. I guess you could use AHCI for a platter, too.