If you have used Azure Functions since the beginning, then chances are you've started with a 1.x runtime. Since 2.x is out, you may want to upgrade to it but will be greeted with the following message:
This helps protect users from breaking their Azure Function. As v1 and v2 runtimes are not meant to be interchanged.
If you still want to do this, then you can simply change the
FUNCTIONS_EXTENSION_VERSION App Setting to
~2 to target the runtime that you want.