Now how can i change my battery

See this Apple article: iPhone Battery Repair & Replacement - Apple Support

If you are going to replace the battery, see -> Get your iPhone, iPad, or iPod touch ready for service

Get your iPhone, iPad, or iPod touch ready for service - Apple Support

The price is approx $ 69.

If you want to know exactly, call Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

or 

call 1-800-MY-APPLE

Tap on "Get an estimate" (after choosing your country) in this support article:--> Choose Your Country or Region - Apple Support

Regards

Giulio

you can’t replace the battery yourself you will need to take your iPhone to a Apple genius service bar to get it serviced by a Apple certified technician that use Apple genuine parts