Factory-built desktops have custom-made motherboards made especially for that manufacturer. Information about what processors are compatible with these motherboards is not published, unlike computers which are built at home or made-to-order using standard retail motherboards.
So whilst the CPU on your Dell motherboard most likely could be upgraded, we have no way of getting any information on CPU compatibility with that board.
If you want the freedom to upgrade the CPU (and most other components) on a desktop computer, you need to have a computer built for you at a local computer shop since they will use a standard retail motherboard for which a CPU compatibility list will be published on the motherboard manufacturer's website for...