Replace the battery

The battery can be replaced by Apple or an Apple Authorized Service Provider.

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

How long does it take?

That depends on your region, here and it took 2 days.

How long? 30-45 minutes, but you need to make an appointment (*) in advance, to avoid queues and to be sure the battery for your iPhone model is available.

Price? Get an estimate using this support page:--> iPhone Battery Repair & Replacement , bearing in mind that the prices are related to US Apple Stores (tax excluded, work included).

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region
• Using the "Apple Store" app, available and downloadable from AppStore.