Generally, when upgrading from .NET Framework to .NET Core you first need to upgrade to .NET Standard (1.x -> 2.x, or directly to 2.1 whichever is least "painful"), and then after that upgrade to whatever version of .NET Core you want to target. Useful links:

• https://dotnet.microsoft.com/en-us/platform/dotnet-standard#versions

• https://learn.microsoft.com/en-us/dotnet/standard/frameworks