Two choices, pound them out or use a press and press them out.
When you "set" a rivet, they swell to fill the hole. They will pound out but be sure to use a punch only slightly smaller than the rivet diameter. If you use too small of a punch, you continue to swell the rivet stem and you will not get it out. You can also use heat and then quench with water to thermally shock the rivet before pounding.