The original Constitution has been changed many times by laws which define it. For example: In 1790, Congress passed a Law which said that only the Federal Government shall make Treaties with the Indians. Prior to that, the Original States had the right to deal with the Indians in their own State and the Treaty power referred to Federal Lands such as the Northwest Territory. The Federal Government usurped the Power of the States at that point. I don't believe "Indian" treaties specifically are even mentioned in the Original Constitution.
The constitution cannot be changed by a law. Laws can be passed to enforce it, or to exercise powers granted within it.
The original Constitution, Art. 1 Sec 10, says "No State shall enter into any Treaty, Alliance, or Confederation;"
This would presumably include with the Indian Nations.
Prior to the passage of Constitution the Articles of Confederation provided: "Article VI. No State, without the consent of the United States in Congress assembled, shall send any embassy to, or receive any embassy from, or enter into any conference, agreement, alliance or treaty with any King, Prince or State;"