If you switch to the Fog of War tool, there are two tabs at the top of the tool - Permanent, and Explorable. If you select Explorable, and then the Add option, you should be able to "add" explorable darkness, which has the effect of erasing areas that have been explored. If you want to erase all of the areas which have been explored, you should be able to "fill page" with explorable darkness. Does that do what you want to do?