Answer by taking a quote from the following article:

La Malinche (Spanish pronunciation: [la ma'lintSe]; c. 1496 or c. 1501 - c. 1529), known also as Malinalli [mali'nal:i], Malintzin [ma'lintsin] or Dona Marina ['dona ma'rina], was a Nahua woman from the Mexican Gulf Coast, who played a role in the Spanish conquest of the Aztec Empire, acting as an interpreter, advisor, and intermediary for the Spanish conquistador, Hernan Cortes. She was one of 20 women slaves given to the Spaniards by the natives of Tabasco in 1519. Later, she gave birth to Cortes' first son, Martin, who is considered one of the first Mestizos (people of mixed European and indigenous American ancestry). The historical figure of Marina has been intermixed with Aztec legends (such as La Llorona, a ghost woman who weeps for her lost children).

The many uncertainties which surround Malinche's role in the Spanish conquest begin with her name and its several variants. At birth she was named Malinalli or Malinal after the Goddess of Grass, on whose name-day she was born. Later, her family added the name Tenepal which means "one who speaks much and with liveliness".  After being baptized, the 20 slave girls were distributed by Cortes among his Spanish captains. Malinalli then took the Christian name of Marina, to which the soldiers of Cortes added the Dona, meaning lady.  It is not known whether Marina was chosen because of a phonetic resemblance to her actual name, or chosen randomly from among common Spanish names of the time. A Nahuatl mispronunciation of Marina as Malin plus the reverential "-tzin" suffix, formed the compounded title of Malintzin, which the natives used for both Marina and Cortes, because he spoke through her.  Today in Mexican Spanish the word malinchismo and malinchista is used to denounce Mexicans who are perceived as denying their own cultural heritage by preferring foreign cultural expressions.  Some historians believe that La Malinche saved her people from the Aztecs, who held a hegemony throughout the territory and demanded tribute from its inhabitants. Some Mexicans also credit her with having brought Christianity to the New World from Europe, and for having influenced Cortes to be more humane than he would otherwise have been. It is argued, however, that without her help, Cortes would not have been successful in conquering the Aztecs as quickly, giving the Aztec people enough time to adapt to new technology and methods of warfare. From that viewpoint, she is seen as one who betrayed the indigenous people by siding with the Spaniards. Recently a number of feminist Latinas have decried such a categorization as scapegoating.

Why was her name changed?
Malinalli then took the Christian name of Marina, to which the soldiers of Cortes added the Dona, meaning lady.