If you have too much, it's a good idea to slice and freeze the part of the loaf of sourdough bread that you aren't going to use. If you have fresh sourdough bread that's gone slightly dry or hard though, it is possible to soften it again.  Wrap it in a paper towel or linen bread bag and give it 10 seconds in the microwave, or sprinkle some water on top then bake it in the oven for 6 minutes at 325°F.