Most all in one are basically a laptop in a screen. Upgrade options are going to be limited if available at all.
There should be a service tag sticker on the machine that will inform you of more information on the Dell website under that model.
I doubt the CPU could be changed at all. You may find an ability to change at least one of the RAM sticks, if equipped that way, and perhaps a storage option. Beyond that, and even if that, it is very likely there is nothing more to do for it.