Can more recent revelation from LDS prophets nullify or change official Mormon scripture? When an LDS prophet speaks, if what he says is different than canonized LDS scripture, do the words of the prophet take precedence?
Are you referring to anything specifically?  If you can provide a specific instance, that might be more informative.

In short: the answers are: Nullify, no.  Takes precedence, yes.

For example, polygamy is an eternal law.  And although it is not practiced today, it has been practiced as noted in the Bible (Genesis 16:3; 25:1; 29:21-30; 30:3-4, 9), spoken of in the Book of Mormon: (Jacob 2:30), and practiced in the beginnings of the LDS church.  However, the prophet in 1890, President Woodruff, wrote a Manifesto which lifted the commandment to practice plural marriage LDS.org topic: Polygamy. In that instance, the prophet didn't "nullify" the law, because those who had practiced it would still be sealed in heaven, however the practice on earth was stopped because the Lord commanded it.

In general, scriptures were written by prophets, who received the revelation from God, and the words of the living prophets are considered scripture: "In addition to [the] four books of scripture, the inspired words of our living prophets become scripture to us. Their words come to us through conferences, the Liahona or Ensign magazine, and instructions to local priesthood leaders. “We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God” (Articles of Faith 1:9)." LDS Gospel Principles

Revelation from the living prophets are spoken as if from God.  (D&amp;C 1:38) Also, God does not change.  (1 Nephi 10:18).  Therefore, the revelation and the scriptures are from God, and God does not change, though he may give some specific instruction to certain people at different times, it doesn't nullify scripture.

This pattern is shown when Christ declared that the law of Moses was fulfilled in Matthew 5:17 Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.

In an interesting address by a modern prophet (an apostle at the time) to the students at BYU in 1980, President Benson said, "Beware of those who would set up the dead prophets against the living prophets, for the living prophets always take precedence." Fourteen Fundamentals in Following the Prophet