Q. How do I cite an amendment to the U.S. Constitution?

According to the Purdue Online Writing Lab, it would look like this in MLA:

U.S. Constitution. Art. I, Sec. 2

U.S. Constitution. Amend. XIV, Sec. 2

The Purdue Online Writing Lab is a useful resource for MLA and APA citation questions. The link also explains how to do a parenthetical citation for the Constitution (and much more).

According to the MLA Style Center page on Documenting Legal Works in MLA Style:  

The section of the United States Constitution is represented in the Works Cited list as follows:

United States Constitution. Art./Amend. XII, Sec. 3.

The intext citation would follow this pattern:

(US Const. amend. XII, sec. 3)

If a constitution is published in a named edition, treat it like the title of a book: 

The Constitution of the United States: A Transcription. National Archives, United States National Archives and Records Administration, 28 Feb. 2017, www.archives.gov/founding-docs/constitution-transcript#toc-article-iv-. 

The Constitution of the United States, with Case Summaries. Edited by Edward Conrad Smith, 9th ed., Barnes and Noble Books, 1972. 

For APA style, follow the advice below from the APA Style Blog - How to Cite the U.S. Constitution in APA Style:

First, if you simply want to make passing reference to the U.S. Constitution in an APA Style paper, you can mention it in text without a reference list entry.

Law students described a great affinity for the U.S. Constitution in their response papers.

However, if you are using some part of the U.S. Constitution as evidence to support a point you are making in your paper, you should construct the citation as follows:

All citations of the U.S. Constitution begin with U.S. Const., followed by the article, amendment, section, and/or clause numbers as relevant. The terms article, amendment, section, and clause are always abbreviated art., amend., §, and cl., respectively. Preamble is abbreviated pmbl. (as in my opening quotation). Article and amendment numbers are given in Roman numerals (I, II, III); section and clause numbers are given in Arabic numerals (1, 2, 3). For parts of the Constitution currently in force, do not include a date. If you are referring to a part of the Constitution that has been repealed or amended, include the year that the part in question was repealed or amended in parentheses.

Here are example in-text citations and their corresponding reference list entries. Note how similar they are:

In text: The founding fathers addressed the process by which new states may join the union (U.S. Const. art. IV, § 3). 
Reference list: U.S. Const. art. IV, § 3.

In text: Women gained the right to vote in 1920 (U.S. Const. amend. XIX).
Reference list: U.S. Const. amend. XIX.

In text: During prohibition, the sale of liquor was made illegal (U.S. Const. amend. XVIII, repealed 1933).
Reference list: U.S. Const. amend. XVIII (repealed 1933).