Congress can NOT amend the Constitution. Read Article V more carefully.
They cannot ALONE amend it. But they can propose an amendment with a 2/3 vote. Then it must be ratified by 3/4 of the states. Or Congress can call a Convention on the application of 2/3 of the state legislatures. But even then the mode of ratification is still proposed by Congress, and must be ratified then by 3/4 of the states. And given that the same people who elect the congressional representatives also elect state representatives, this is not likely to make a big difference.