Help Refactoring with Roslyn

I am trying to build a Refactoring VSIX with Roslyn for VS 2015. I have all the logic working but I am missing one important step. How to I replace the new code over the old code.

I have TargetStartment as the original piece of code I need to replace String.Format("Hello World {0}, x)

Dim TargetStatement = as InvocationExpressionSyntax = CType(ParentToken.Parent,  InvocationExpressionSyntax)

And I have a String with the replacement Code $("Hello World {x}")

The few example are very unclear on how to do this simple refactoring.