Simply extract the compressed "Pop The Lock Template.zip" file with whatever unzip application you wish. I personally use "The Unarchiver" which is free to download via the Macintosh App Store.
Once extracted, simply open the ".xcodeproj" folder which will be located inside the extracted Pop The Lock Template folder. Once opened, XCode will run and the project will open.
Here, you can access the games main code when clicking the GameScene.swift file to the left of XCode. The code doesn't require too much complex code so even beginners will be able to pick up on what line does what pretty quickly. Like stated above, do feel free to edit any code.
If you'd like to edit any of the games graphics, simply open the "Assets.xcassets" folder to the left of XCode, under the GameScene.swift file. Here, you will be able to see all of the graphics used in the code. If you wish to edit any of the graphics, please make sure that you use .png files and name whatever you're replacing it with, the exact same file name as the original name. e.g if you want to change the Player graphics, replace it with a another .png file and still name it "Player.png"