The Talmud Yerushalmi comments in the beginning of Masechet Megila that strictly speaking, one can fulfill the obligation of Megila reading already from the beginning of the month of Adar, through the 15th of the month. Although Purim is observed on the 14th (or, in some locations, on the 15th), nevertheless, one can read the Megila and fulfill this Misva already from the beginning of Adar. The Yerushalmi infers this Halacha from the verse in the Megila that speaks of "Ha’hodesh Asher Nehpach Lahem Mi’yagon Le’simha" – "the month which was transformed for them [the Jews] from anguish to joy." This indicates that the entire month is given a special status, such that the Misva of Megila can be fulfilled already from the beginning of the month.
On the basis of the Yerushalmi’s comment, the Abi Ezri, cited by Maran in Bet Yosef, writes that if a person knows ahead of time that he will not have access to a Megila at any point during Purim, then he should read the Megila earlier in the month of Adar. For example, if a person will be traveling, and on Purim he will not have access to a Megila and cannot bring one with him, then he should read the Megila before Purim, anytime from the beginning of Adar, in order to fulfill the Misva according to this view. He should read the Megila both at night and during the day, and no Berachot are recited over the reading. On the 14th (or 15th) of Adar he should fulfill the other Misvot of Purim – the meal, Mishlo’ah Manot and Matanot La’ebyonim – even though he will not be reading the Megila. This ruling is codified in the Shulhan Aruch (Orah Haim 688:7), and in Yalkut Yosef (listen to audio recording for precise citation).
Summary: If a person knows before Purim that he will not have access to a Megila on Purim, then he should read the Megila at some point from Rosh Hodesh Adar, without the Berachot, at night and again during the day. The other Misvot of Purim should still be observed on the 14th (or 15th) of Adar.
