GO WEST YOUNG MEN AND WOMEN: The Upper West Side is abuzz with New York Fashion Week at Lincoln Center, and the week-long event seems to be rubbing off on the neighborhood. A bevy of stores is getting situated in new digs. Theory recently moved two blocks south to 201 Columbus Avenue, with Helmut Lang taking over its former spot at 230 Columbus Avenue, and Athleta started welcoming fitness-minded shoppers at 216 Columbus Avenue. More outdoorsy types can start casing kayaks, getting bike tune-ups or planning their next travel adventure at the new Eastern Mountain Sports that bows Friday at Broadway at West 76th Street. And designer shoppers are eager for the debut of Century 21, which is busy filling racks in a 61,000-square-foot space at 1972 Broadway.