My iPhone 6 battery dying at 20% Or more

Hello Apple People,

I have heard this being a problem with the iPhone 6S, but mine is just a 6 and it will shut off even when there is still even up to a 50% charge sometimes. My phone's OS is up to date but still nearly every time its gets down to 20% it shuts off, then sometimes (much more rarely it will shut down at up to 50% charge remaining) but almost every time it gets to 20% I have to either be next to a wall, hope I'm in my car, home or have a battery pack while on the go!

Can I take my phone in and something be done (early upgrade) like people with the iPhone 6s can, or am I just going to have to wait till my upgrade and have to deal with this? I dont have an upgrade for a while and this has had a big impact on work/being a professors aid and being on the go.

Thank you

Cemey