Battery replacement on old iphone 4

hi, i've had iphone 4 since first day of launch june 2010 think battery starting fade little. it's lasted whole day seems getting 20% 6pm @ night never used to. since charge every night it's possible it's running out of life.

know if it's possible replacement battery in apple store without replacing whole phone? if so, how cost? (i'm in uk)

cheers!

 

after reading these first 2 posts think i'm seeing double.

 

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