Please note that this process will add a third party TurnKey repo that I personally manage. I had to do that as I do not have direct access to the offical TurnKey repos and we need updated packages for a Stetch base.
Starting point: v14.2 TKLDev running in a VM (or bare metal if you prefer...), logged into terminal as root.
# checkout appropriate branch of common and core
cd common
git fetch origin # assumes you stil have turnkey as origin - can be skipped on fresh v14.2 install
git checkout -t origin/stretch-dev