There is actually a process defined in Constitution that allows for a Constitutional Convention. It could be called by 2/3s of both houses of Congress or (and this is important) 2/3s of all states.
2/3s of all states is 34. Republicans control (right now) 32 State Legislatures. Arkansas has called for it to deal with Gay Marriage and Abortion, but it doesn't really matter why you call for it once you 34 states to do it. The delegates can then right anything they want.
Now, if they want to Amend the Constitution, you will still need 38 states to approve the Amendments. But the Constitutional Convention can write any rules they want. The last tme we had a Convention, they threw out the old Constitution ("The Articles of Confederation") and wrote an entirely new one. Or they could lower the number of states needed to approve an Amendment.
It's scary because many people want to call it for different things. Jeb Bush wrote in November of last year (link),
"Republicans should support convening a constitutional convention to pass term limits, a balanced-budget amendment and restraints on the Commerce Clause, which has given the federal government far more regulatory power than the Founders intended."
Even many liberals support the idea to overrule Citizens United.
But letting every state get together, who knows what they would come up with. They wouldn't be limited to any topic once they are all together. That is a little worrisome.