is it stuck? whatever you do, don't stick the button all the way on the tip and pull. i did this once and pulled the tip out but once it was out i couldn't get the tip off the button! you can use the notch on the button and pull but don't force it all the way in. if that doesn't work you can approach it from the inside and push with your thumb. once i had to take a bottle cap and use it to push from the inside because it was stuck too much and the tip hurt my thumb. you could even use a blunt object like a wooden dowel. just hold he top upside down with the cap off and a dowel on the inside on the tip. bang the dowel on a hard surface and viola! watch for flying tips, though. it's safe, but just barely.
good luck.