By using the Trainz-build tag and the proper number for the version you are setting. If you use CCP it will be done automatically, and then it will choose the build number of whatever version you are using (ie:2010) unless you change it manually.
Of course whatever version you choose, you have to be sure the content is properly made for that version. I believe the content that is being revised for the DLS is made using Trainz-build 2.9