The notion that because God knows your future, your fate is already decided is known as predestination (or predeterminism in philosophy). According to Mormons, predestination is false doctrine.
Let's say that according to Heavenly Father, I am not going to the Celestial Kingdom. He already knows that.
This is the premise of predestination which collapses in LDS theology. The original doctrine from the books of Genesis, Moses, Abraham, and 2 Nephi, and several epistles in the New Testament, declare that man was given to be an agent unto himself.
God does not choose whether you go to the Celestial Kingdom. You do, based your choices according to the Law of the Gospel. If you live a celestial law, the scriptures say, you will be risen to a celestial glory.
In this sense, God's foresight has nothing to do with determining one's salvation. If it did, we would not need the Plan of Salvation or the Atonement, or even earthly experience: our choices would not have any meaning and the Law would have no effect on us... everything would fall apart! In fact, this was precisely Lucifer's plan (Moses 4).
How do we change what he already knows?
Latter-day Saints, like most Christians, believe that God is omniscient: He knows everything; we know so little. Who are we to change His mind? I think, rather than changing what He knows, we would do better to pursue knowing what He knows.
How do we Change our Determined fate based on what Heavenly Father already knows?
As President Monson says, "Decisions determine destiny." (Read that whole talk, it's very relevant.)