A pair of pliers can do it if you don't have the wind back too, but expect some war wounds as they tend to slip :homer:.
What you need to do is get some large(ish) pliers and hold them in your hand so that they are held open at the width of the piston. You place the jaws of the pliers into the two notches in the edge of the piston and twist clockwise. This will wind the piston back in. It can be hard work and frustrating, especially if you're calipers have seen better days, but it saves spending money on a tool you'll use once every couple of years.