You can't, it's all mixed in together. The only way to make a cake with less sugar is to make it from scratch, but if you just reduce the amount of sugar in a recipe it probably won't turn out the same. Making a cake with less sugar doesn't make a lot of sense to me - why do you want to do this?
You CAN reduce the amount of sugar in the mixture by buying some of its fresh ingredients and adding some proportion to the mixture (e.g. adding some amount of self-raising flour, egg, and water). You will make a lot bigger cake and less sweet.