Use the strap tool included in the bottom of your watch case for changing both leather and mesh straps. You can also use a similar, small item with a narrow point (like an earring or a needle) to change or adjust the size of your strap.
Use the narrow end of the tool to push the spring bar through the hole from the side of the lug. When pushed, the spring bar will loosen from the hole and can easily be detached from the other lug.
Detach the strap from the watch and remove the spring bar from inside the strap. The same bar will be used to attach the new strap.
To attach a leather or a mesh strap, push the spring bar through the hole in the watch strap until both ends are visible.
Place one end of the spring bar in the hole in one lug and push the other end into the hole in the second lug to attach the strap.