I had an iPhone 5S whose battery life had depleted over a few years of constant use to a fraction of its original lifespan. You can buy a kit online to replace it yourself for about $30, takes a week to ship from the US, and it's a fiddly, complicated, laborious job that requires good hand-eye coordination, which I do not have. Looking at reviews of the kit, a lot of people manage to do it just fine, but a lot of people manage to break their phone in some substantial way too.
Found iRepair.ca on Yelp, seemed most people were happy, so strolled in on a Saturday afternoon. Dropped off the phone and they were done less than 45 minutes later for $50 plus tax - well worth the extra $20 to get it done by a pro on the spot, and my battery life is vastly improved. Service was particularly friendly too! Would definitely recommend to anyone needing iPhone repair.