How to Keep Your Cool: Reduce summer overheating using floors
This is only going to be a very short post, as unfortunately there's not a lot that can be done about floors to reduce overheating. In fact, insulating under floors, while a great plan for most of the year, will tend to increase overheating a little bit, since the ground is a great heat sink.
If you're lucky enough (in this context) to have uninsulated suspended timber floors, have a look at the information on passive stack ventilation here.
If you have solid concrete or other hard floors which are on the ground, removing any easy-to-roll-up rugs or carpets is likely to help a little more heat pass into the ground and keep your toes cooler at least. Upper storeys of houses or apartment blocks aren't likely to get much advantage from this, except that hard flooring conducts heat better than carpet so tends to feel cooler as it moves heat away from your feet faster than carpet does.
Otherwise, keep your energy focused on the parts of your home that the sun can hit, and remember to keep shading sunny parts of your home!