Azure Functions 2.x runtime is based on .NET Core 2 from .NET Framework 4.7 in 1.x. So, any application migrating from 1.x to 2.x needs to make sure that they are using and referring the compatible code and libraries.
We can also update our Azure Functions to target the appropriate runtime using the below steps à
Inside Azure Portal, select the Function App, click on Application Settings within Platform Features tab.
We can specify the version number for the APP SETTING NAME – FUNCTIONS_EXTENSION_VERSION
~ tilde à indicates that the latest version of that major version.
Get all the details here
Hope it helps..