It depends on which part of Epcot you're already in. If you're in Future World, either take the bus from the front of Epcot to AK or get a taxi. If you're in World Showcase, exit Epcot via the International Gateway and go to either the Beach Club or Boardwalk and take the bus from there or get a taxi.
The taxi will be the fastest, but it'll cost between $15 and $20 plus tip. The bus isn't to bad (note that the AK route is shared with Blizzard Beach).