Look at the chuck that connects your pump to the valve stem. Most pumps convert like this:

1. Unscrew the end of the chuck.
2. There are two parts inside. You might have to work at it a little bit to get them both out. One is a neoprene gasket, the other is a hard plastic thingie.
3. Turn both parts upside down (individually) and put them back in the place where they were.
4. Screw the end of the chuck back on to hold everything together again.