I'm not sure what the start-stop system is, so my comments below do not reflect this option.
I just replaced the battery in my 2016 CX-3 GT yesterday because it was noticeably slower to crank the engine over in the mornings. The replacement battery on the CDN version is a Group 35 (this represents the overall dimensions of the battery case), with a minimum of 520 (CCA) cold cranking amps. I replaced mine with a slightly higher rated battery with 640 CCA, because Canada!
I've replaced batteries in every vehicle I have ever owned, plus a few in my kids vehicles so it's not a difficult job. Because this vehicle has many electronic options that I've never had before, I decided to watch a youtube video and learned that if you connect a battery tender to the terminal clamps BEFORE you remove them from the battery, you will not loose your radio presets. It also allows other information stored in the onboard computers to be retained. Apparently, if the radio looses power it might not work until reset with a code. By using the battery tender I was able to replace the battery without any problem and everything works as it did before.
It is an easy job and all you need is a 10mm deep socket, since the bolts on the battery hold down clamp and terminal cables are the same size. Remember when removing the cables from the battery it's negative first, then positive. When you reinstall attach the positive first, then negative.