Take line 1 or line 2 to the central train station (25-30 mins). To the second floor, go outside to the right, there is a bus terminal (about 100 meters away from the station). Take Bus 601 to Linz airport (25 mins). The whole trip will cost one hour or more. The periods of the tram and the bus are 20 mins and one hour, respectively. The time table of the bus can be found on the website of the airport.
JKU->Linz Hauptbahnhof->Flughafen Linz
JKU-> Linz centrall station-> Linz airport