How do you overwrite a current Minecraft game with a previous save file?

Question

I died in lava while mining and all of my items burnt. I haven't respawned yet but I know there is a way to somehow copy a Minecraft save file directly into the game so I can play from that save. I'm not sure how to do this but I know where the save files are located. How do I copy the save into the game? This is for PC and it runs on Windows 7.