These kits are ideal if you are comfortable opening your device. They typically include a new battery, specialized screwdrivers, and adhesive strips. iFixit iPhone 6 Replacement Battery Kit Go to product viewer dialog for this item.
SOLVED: Battery Replacement - OEM Specs? - iPhone 6 - iFixit where to buy an iphone 6 battery
If you want to avoid the risk of damaging internal components like flex cables, professional services are the safest route. These kits are ideal if you are comfortable
A budget-friendly option from repairpartsusa.com priced at $10.79. It includes a zero-cycle battery and a lifetime warranty on parts. NewPower99 IPhone 6 Battery Kit where to buy an iphone 6 battery