Neck Splits

Dig around. Find a small cardboard box. Write on it, "good brass gone to brass heaven". Lay it gently in there. It will be joined by others. That brass can be a lesson to people just starting down the road to perdition - er, just starting to handload.

You can't fix it. The crack is created when you fired brass that was forced to expand beyond it capacity. Firing hardens the brass so the capacity for hard brass is less than the capacity for soft brass. Annealing softens the neck brass, it stretches better, thus restoring its ability to expand. Buy an annealer or find a service or buy new brass. You might check how much your dies size the neck. Maybe you are oversizing the neck and the work-hardening is on you.
 
Having new Winchester brass get split necks on the first firing caused me to buy an annealing machine. And no, you can't fix the ones that are split but you may can keep it from happening so quickly.