If you are referring to a standard 6P2C or 6P4C modular plug (see RJ11 at Wikipedia: Registered Jack), you don't. You cut the old plug off and crimp a new one on with a crimp tool (example: Ideal Telemaster) designed for the plugs you are using.

If you are using another type of plug or perhaps outside the U.S. post back with the specific type of plug in use.