1) download the latest flash develop if you update to a major version so you can  select from the dropdown the appropiate air sdk version

2) download flex sdk . unzip it somehwere

3) download air sdk . unzip in the same folder you choosed at 2) . overwrite when asked .

4) add the folder to the flash develop settings (you can right click your project -> properties)

5)  if you get the “unknown namespace” error when building the project check 2 things:

a) open: bat\SetupSDK.bat

set this var to your desired folder set FLEX_SDK=C:\Program Files (x86)\FlashDevelop\Tools\flexsdk

b) make sure you don’t have in your enviroment variables in windows in the “path” variable a path to your older air/flex sdk . Delete that one and add the new folder you just used for the combination of flex/air sdk.