It would take an amendment to the constitution, virtually a miracle.
An amendment to the Constitution is correct. In order to do this, 2/3 of both the House & the Senate would have to pass the amendment and then send it to the state legislatures for ratification, which requires 3/4 of them to do so. There is no Constitutional time-limit on this, but since the ERA amendment was sent to the states (and failed), Congress has traditionally set a time-limit of 7 years on the process.
There is a series of four essays on term limits at this blog site that you might find of help and interest, as they deal with what the founders thought and debated over this topic: http://anantifederalist.wordpress.com. The fourth essay in the series deals specifically wtih your question: http://anantifederalist.wordpress.com/2010/02/14/congressional-term-limits-yes-we-can-hope-for-change/
A vote by Congress, which is highly unlikely.
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them