From centrum to Santasport and back you can use a public transportation. Take a public bus number 5 from Ruokasenkatu in the centrum. Please note that there is a bus 5 going two different directions, you need to take the one going to Ounasrinne. Hop off from the bus on the street called Porokatu. That is the closest bus stop to Santasport, it is about 200 meters away.
You can check the timetable for buses from here.
Since Santasport is only 2 kilometers away from the centrum, it is easy to access by foot in 15-20 minutes. Biking takes less than 10 minutes.
If you are coming from railway station, airport or from somewhere else more far away, the best way is to take a taxi.
The distance between Rovaniemi Airport and Santasport is 10 km.