Visas are a form of entry permit issued by one country to the citizens of another. Your visa stamp can expire while you are in the U.S. This is very common and there is nothing wrong when this happens. There is no procedure to renew visa stamps in the U.S. because of the fact that a visa stamp does not have to remain valid while you are in the U.S.
You are not required to travel outside the U.S. just for the sole purpose of renewing your visa stamp. However, if your visa stamp is expired and you travel outside the U.S. you are required to renew your visa at a U.S. Consulate or Embassy in order to re-enter the U.S.