Replacing an Iphone 5c battery?

For the past few months my phones been dying at 15 percent and recently as high as 28 percent. I've noticed that it's battery life has also been getting shorter. I used to be able to go 2 days without charging, and now I find I have to charge it twice a day. I've had my phone for about 2 or 3 years so I'm not sure if this is a normal thing or not. I'm thinking the battery needs to be replace.

I originally got my phone from Verizon but it's an apple so does it matter what store I go to to get it fixed? Which would charge less? I've seen relatively cheap kits to do it yourself, but I'd probably just ruin my phone instead. Would I be better off replacing my phone? Replacing is honestly the last thing I want to do, call me overly attached, but if a new battery cost the equivalent of a new phone then I might as well replace the entire phone.