01 x5 how to change thermostat

[yenanx5]

I just bring my car to the dealer to have the service light on. They toll me need oil change, and AC service, and replace tensioner and pulley and belt. It was cost me $1800. advicer was tell me every thing is ok, and he also check multi point and every thing ok.
One day later, the service check soon was on. I bring my car back to the dealer. He toll me, he will put on computer for free normally cost $100. I wait for 1 hour later. He came out and toll me that my car need change the thermostat and secondary air fault. They will charge me $1000 to fix that. I known it was a riff, but I could not say any thing.
Lucky I found this website for hlp.
Please anyone know what is the second air fault is, could you post the pics so I can DIY. And how to replace the thermostat Please post pics.
Thank you