how do you legally overthrow the us government?

By voting?

People did that. It didn't work. United States citizens should be
very, very concerned. Bush DID NOT get the popular vote in 2004.
Diebold voting machines took  Kerry votes and gave them to Bush. I
live in an Ohio county that is very Republican. In 2004, we voted 63
percent for Kerry. We had paper ballots. Many Ohio counties, very
Democratic, went towards Bush. They had Diebold voting machines.
Americans need to wake up and wake up quick. It's not a Republican
thing or a Democrat thing. We must stand up against a dictatorship,
the likes of which the United States has never seen before.

Ah, yes, the "Bush as evil dictator" theory. Up until now, Lyndon
Johnson and Richard Nixon were the most maligned U.S. Presidents of
recent times. I guess it's always gonna be someone.

Regarding "Bush DID NOT get the popular vote in 2004."

At the risk of adding fuel to the fire of an off-on-a-tangent
conversation, I just gotta respond to this by saying: "So?"

The president of the United States is NOT elected by the popular vote.

Now, if you want to argue that faulty technology or other voting
irregularities resulted in key states improperly allocating their
presidential electors to the U.S. Electoral College, go right ahead.
But if you want to seriously question the validity of a presidential
election, you'd probably be more effective not invoking the popular
vote at all...

The constitution only allows a few ways to legally "overthrow" the government.

1) Voting (probably not the answer you are looking for)
2) The president, Vice-president and any Civil Officers can be
impeached and removed from office but that would require the 2/3rd of
the senators and then the President can only be impeached for treason,
bribery or high crimes)
3) Amend the constitution in a favorable way to support a complete
change of government (probably not very viable because 2/3 of congress
will need to support it as well as 3/4 of the states legislatures or
through 3/4 of the states through constitutional conventions)
4) Amendment 24 allows the vice-president and a majority of either the
principal officers of the executive departments or of such other body
to vote the President as incapable of dischartging his duties and then
the Vice-president becaome acting president.  This is not as easy as
the show "24" led you to believe, if the President decalares in
inability exists then the issue goes to congress for a vote needing
2/3 majority.
5) Probably the most popular is to get yourself a very big and loyal
army and take the government by force, install yourself as president
and write a new constitution and rewrite the laws and history books to
validate your take-over as legal but this is more of a win on a
technicality.

It Depends on who is trying to overthrow the government. It also
depends on what part of the government.  You could overthrow the
Executive Branch, but the Legislature and Judical Branches would
remain.  The only way an outside force could overthrow the government
would be through war or corruption.  Seeing how the US has the
strongest army in the world the probobilliy of that happening is very
low.