You would be better off selling it and buying a new system.
To replace the CPU in a pre-made PC is dicey. Dell's aren't crap, but there is no guarantee that any other processor you drop in that socket is going to work.
You can't replace DDR3 with DDR4 unless the PC has a motherboard which supports both. That would likely require a new motherboard.
You can't put every motherboard in a Dell because they probably won't line up.
What you can do, is contact Dell and find out what upgrades they offer. If they offer upgraded processors, those processors can likely be dropped in to your motherboard and work. Similarly there might be slightly faster DDR3 ram which might work in your MB.