The current wording of the oath of office is:
I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter: So help me God.
This is used for members of Congress, and for military officers. It is specified by 5 USC 3331. This wording was changed in 1862 to make it stricter, in the face of fears of disloyalty on the outbreak of the US Civil War. The language was further revised in 1884, and again in 1966. See "Oath of Office" an official page of the US Senate, and "Oath of Office" an official page of the US House of Representatives.
However, none of these clearly define the precise meaning of "support", which has ben part of the oath since the version prescribed by the First Congress in 1789.
I am not aware of any legal case in which a person was charged with having failed to support Constitution, and so no court interpretation of this wording seems to have been made.
It would seem, by the "ordinary meaning" rule, that a person who vows to "support" the Constitution is promising to adhere to its lawful commands, and to be loyal to the government established by the Constitution. It does not include a promise not to advocate for lawful changes in the Constitution, nor would it be violated by expressing the view that changes in the constitution would be improvements.