Formalities about my son's new passport

Hi,

I've got my son's new passport with his new last name. Now I asked my HR regarding on how to change my son's name on his iqama with his new passport name. They said that they will give me a letter for changing the name and bring it to Jawazat Office in Riyadh along with the necessary documents of my son.

My question is, am i allowed to go personally to Jawazat office and get it done? because they instructed me to do so and how many days it will take to get the new iqama and lastly, how much it will cost me?

Thanks