Get the tips of your pointer fingers slightly under the top edge of the key, press down on the bottom of the key with your thumbs, and with a firm and somewhat forceful motion, pull the key back very quickly from the top. It's like removing a bandaid -- do it quickly. If you do it slowly, you run a higher risk of damaging the internal components.
Most of the spacebars have a couple metal bars running the length of the key. Their ends need to be secured in tiny holes at the base of the spacebar area, however the metal bar, especially the top one, often comes loose when pulling the key back. Make sure the ends are properly seated in the holes, and make sure the upper bar is flipped upward so that it runs against the top of the spacebar area (if it is in the downward position, it will not fit into the notches on the underside of the spacebar). The bottom half of the spacebar can remain connected while it is pulled back from the top. As for working inside the key, I prefer using tweezers, because other instruments tend to be too blunt.
Once everything looks good, position the spacebar back in place, and press it firmly downward with several fingers. Press your finger across the entire length of the key in order to make sure all of the clips have secured themselves.
Please note, there is always a risk of breaking some of the small plastic clips when removing a key. no matter how good you get at it, you will probably break a key every now and then. Although, like I said, pulling it back very quickly helps reduce the potential for damage.