The Founding Fathers of the US knew that the world would change, and people would need more and more new things and rules. That's why they created the Constitution changeable. The Constitution of the US may be altered by proposing new amendments. The process of introduction is quite complicated and is contained in the Constitution itself. It is described in Article Five, and many Americans are familiar with it. Still, the process remains rather difficult and requires a lot of effort.

Amendments can be proposed in Congress when 2/3rd of both Houses agree. The states also play an important tole in the process: 2/3rds of the state legislatures must call Conventions to propose amendments. Whether amendments are first proposed by the states or Congress, 3/4ths (or 38 states) must ratify (or approve) them before they become a part of the Constitution—the Supreme law of the land.

More Info: billofrightsinstitute.org