If your device's battery can no longer hold a charge, you need to have it replaced. Use one of these options:
Within warrantee period
If you are still within Apple's one-year limited warranty, or have purchased and are within the extended AppleCare Protection Plan, Apple will replace your defective battery. (They define "defective" as a battery that drops below 50 percent of its original capacity.)
If your device is no longer under warrantee, you can use Apple's Battery Replacement Service and pay a flat fee to have your battery replaced.