Just lay it flat and blow it?
Not just that, applying heat will increase the natural tendency of the plastic to warp even more.
I usually apply heat on the underside with a hairdryer for about... 10 seconds?, and then I flip it and then force the new shape by pressing it on both extremes over a flat surface (if you have an eye for manual works, you can do it just with your hands). Be careful with the sides and don't apply pressure on them if it's not needed, just be sure they are not warped too.
Then you can wait for it to harden or apply cold water over it.
If it's still warped, you can repeat this procedure again.
I've done this procedure with dozens of keys, and not just spacebars: Sometimes other keys like right shift, enter, numpad + and numpad enter also are slightly warped. For smaller keys you need to apply more force though.