You need to replace with the same fluid type that is already there. Unless you are also rebuilding the entire system. When I rebuilt the brakes on my old cars, I went with all new rubber stuff and went with DOT-5 (Silicone). It was a good choice for me as I use my collector cars (don't go road racing where heat can overwhelm silicone). I've not had to flush or bleed the brakes in decades of light use. Dot-5 doesn't absorb moisture so my 25+ year old fluid still looks clear and functions fine.
But if you are simply flushing the system, use the same fluid in it already, most likely DOT-3.