NATO straps come in a variety of colors and can change the whole look of your watch. The first thing you'll need to do is get a Nato strap and a strap-changing tool. (included)
Then put the tool in between the strap and the lug of the watch. Press gently and pull towards the strap. The spring bar fall out and the strap is free!
Put the spring bars back into place and thread your new NATO strap in between the watch and the spring bars as is shown in the picture below.