Bleeding cooling ststem

[Dee4Life]

I have an issue with my 140 bhp 2.0 diesel DSG not reaching 90 degrees. Based on other threads in here the mechanic changed the DSG inline thermostat. However the temp needle now hardly moves at all. Before I go to the hassle of changing the main thermostat I read on a forum https://www.pistonheads.com/gassing/topic.asp?t=1610899 that the system may need bled of air. When mechanic changed the inline one he just nipped the pipes so some air could have got in.

Query is how easy is it to bleed the cooling system? Is there a 'bleed valve' or do you just loosen pipes until you see air turn to liquid?

Would you normally change the thermostat or the temp sensor first?

Any advice gratefully received .