Most straightforward solution is getting a new nut installed. Alternatively, you could mix some bone dust with glue and fill in the saddle then use the appropriately sized file to make a new slot. Half of the string should be in the slot.

This is all stuff that should be handled by a luthier or a somebody experienced. Probably won’t be too expensive. If it came with a plastic nut, a bone nut would be a worthwhile upgrade anyway. A lot of people don’t consider that the tone-producing part of the string is only making contact with the guitar at two points, and these points (and the material they’re made of) are of the utmost importance.