Hi, all.
After a year use, I think the phone battery is coming to its day....

at first, a day usage of BT headset, some Edge for email checking on the road, wi-fi and many phone calls would last full day and had some extra next day even I didn't charge over night. Now, I get a warning of 20% remaining around 7pm....which is about 8hour use.....thus I think battery is dying.
I am checking on line to see if I can change battery myself by getting battery on line or if the apple store has battery replacement service available.
Thanks in advance.