When you change the Rendering Mode, Unity applies a number of changes to the Material. There is no single C# API to change the Rendering Mode of a Material, but you can make the same changes in your code.
To see the changes that Unity makes when you change the Rendering Mode:
switch (blendMode)
{
case BlendMode.Opaque:
// Changes associated with Opaque Rendering Mode are here
break;
case BlendMode.Cutout:
// Changes associated with Cutout Rendering Mode are here
break;
case BlendMode.Fade:
// Changes associated with Fade Rendering Mode are here
break;
case BlendMode.Transparent:
// Changes associated with Transparent Rendering Mode are here
break;
}