Changing the strap on any Apple Watch is pretty straightforward once you know how. Here, we've got an Apple Watch Sport, whose box contained a Medium/Large strap as well as a Small/Medium strap.
You can see which size is currently attached by looking on the back. There will be a tiny circle at the top of the strap with M/L or S/M.
If you find that it's too big or small, you can swap the straps. You'll only get one half of the strap in the box - it's the portion with the holes in - because that's the only one you'll need to adjust the size.
Simply turn the Apple Watch over, and press on the small button that's situated above the portion of the watch we're swapping. Now, slide the strap away from the watch to remove it. It's just a case of sliding the new portion of the strap into the watch face until it feels solidly in place.