The original battery in my iPhone 6 was on its last legs. From a full charge, the phone would suddenly die after 20-30 minutes of moderate use. The charge percentage indicator had become completely unreliable.
All the steps of disassembly were frightening, but the only part that actually presented any difficulty was removing the adhesive holding the old battery in place. I got about a third of the first strip out before it wrinkled and broke off; the second strip snapped off almost immediately when I tried to remove it. I didn't have an iOpener or other adequate heater, but I eventually got more than half of the remaining adhesive out using plastic cards. I then pried the battery out by force, which was successful, though the battery bent and rapidly heated, and I was surprised the frame of the phone was not damaged. The remaining steps went smoothly, and the phone is now as good as new.
Do not skip the steps involving detaching the front and rear assemblies prior to removing the battery. Take your time with the adhesive removal, beginning as gently as possible.