Google this page is not a duplicate
Replace broken key
Use a small screwdriver or knife to prise off another similar sized key on your keyboard (most user never use the windows key, bottom left between <CTRL> and <ALT>)
Place the screwdriver or knife tip under the bottom edge of the key and gently lever it upwards until it clips off.
Now put this key over the position of the missing key and gently press down, it should clip on successfully