Same way you repeal any other law, by repealing or superseding it. If the English government of the time had the authority to make a law, then the English/British governments of later times have the authority to make other law, with the effect of repealing, superseding, or changing the Magna Carta. How could it be otherwise? Did the nobles and king of 1215 somehow have some form of sovereignty that was superior to the sovereignty the queen and Parliament have in 2013? Does the mere insertion of "forever" into a law mean that it can never be changed? If so, what happens when the whole country and every member of Parliament agrees that a law is outdated and wrong, but that law has the word "forever" in it?