how can I change the nove version from 16 to 14 or 12 on my project?
Not possible at the moment (using provided buildpacks). If you want, you can provide a Dockerfile and use Dockerfile application mode, there you can use any node version you want.
This post can be a solution to your problem:
I have specified Node 10 in Dockerfile and it works flawlessly.
In Dockerfile, just add this line and change 10 to any version you want.
thanks a lot for your help