If you are referring to a standard 6P2C or 6P4C modular plug (see RJ11 at Wikipedia: Registered Jack (http://en.wikipedia.org/wiki/Registered_jack)), you don't. You cut the old plug off and crimp a new one on with a crimp tool (example: Ideal Telemaster (http://www.idealindustries.com/prodDetail.do?prodId=30-696&div=2&l1=tools&l2=crimpers&l3=30-696)) 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.