MBP first generation Service Battery
Battery degradation is normal, and seeing that your battery is almost 4 years old has went through 820 cycles, you battery retention is still quite good. Degradation of battery is irreversible.

If you really want to get a fresh battery, you can take it to an Apple store. It'll cost somewhere around $150.

Apple replace defective notebook battery only within 3 years of purchase with AppleCare, so you wouldn't get that benefit.