Density is a bit tricky, since it's mostly an inferred value. Given a specific mass and radius, the Density is just a product of that.
Since we simulate the radius of a body, by taking into account the compression forces of gravity vs internal heat pressure, we can determine the proper radius for a body like Jupiter. If you want to be able to enter anything you like into the density, you'll need to turn off the simulation of radius. You can do that by going to the materials tab, and looking for the switch "Simulate Radius" turn that to off.