Why replace at all?
Generally steel tips are very durable and it takes a long time to wear them out, at least if you don't sharpen them all the time. But nevertheless they might be worn out.
Another reason to replace them might be that you prefer longer tips, to group better or because you want those grooved points for more grip as you hold your darts with one finger at the tip.
That's what you'll need:
Screw the screw into the barrel. It is very important that it fits exactly into the barrel and is screwed in straight; otherwise you might damage the barrel.
Now clamp the old point into the vice. It should be very fixed and not be able to slip out.
Then you grasp the screw with the pliers.
You beat with the hammer against the pliers and the tip should separate from the barrel.
Put the new point into the barrel, but the dart upright with the tip pointing down on the plastic sheet or leadblock and with a beat of the hammer on the screw(don't beat too hard) the tip will be fixed into the barrel.