How do you fix script nut on skateboard?

I'm assuming you mean a stripped nut? If so, it's best to simply replace it. They cost about 50 cents. If your axle or kingpin is stripped, you need a tool called a re-threader. Simply apply the threader on the axle and retool the metal to work for the nut. If your kingpin is stripped, I'd suggest getting a new one, or possibly getting a new baseplate/kingpin combination. Otherwise, get new trucks!