Which transport should i take to Kunming Train station from the airport?

You can take the direct airport shuttle bus line 6 to get to the Main Railway Station. The duration is about 45 mins in usual time. The ticket fare is CNY 25 per person.
In addition, no subway at the airport can take you to there directly. If by taxi, it costs CNY 80 -90 in the day time. And the duration is about 35 mins.
You can choose the way you like. :)

Why do you say line 6 when list above says line 2 goes to railway station? I'm confused which is correct...

Oh, nowadays, both airport shuttle bus line 2 and line 6 can take you to the Main Railway Station. So don't worry. Just tell the airport staff your destination. :)