There are three ways you can soften butter.  The first is to let it sit at room temperature.  This is the best way, but also requires the most time.  If you are in a hurry, you can also cut the hard butter into uniform cubes and place in a bowl of warm (not hot) water.  Let it sit for a few minutes, then drain the water and then stir the butter. The third way is to cut the butter into smaller pieces, place in a bowl, and microwave for 5 seconds at a time.  Stir the butter between each interval.  This method requires you to watch the butter closely.  If the butter melts, you'll have to brush on the butter with a pastry brush.