help: how to change OPT start date?

My friend he mailed his OPT application yesterday. The start date is in the middle of July. But he just realized that H1b will gone soon and he is still looking for job. Because the grace period is only two month, he need to change his start date to after August to make sure he does not need to leave USA in case he can not find a job in the next few months. My question is:

1. Since he mailed his application using regular mail, how can he get his mail back?

2. If he cannot get his mail back, how can he change the starting date directly immigration office?

Thanks a lot.