EDIT: This is for the microwave btw
Heat it with a cover that will keep steam in! I.e. not just a paper towel. If you have a microwave safe tupperware container you can use that with the lid popped open in one corner to vent. If you just have a bowl find a plate that'll fit on top of it decently. Your rice won't be as good as fresh but it won't be all dry and icky as if you haven't covered it.