Pattern search in a string and replace with *sometext* | VB.NET

I'm new to VB.net development, as i'm making an application, i'm wondering how to search for a specific pattern and replace the result with sometext

eg.

original string : the name of my application is abcdef_ghi. its my first project

Pattern: abcdef_

replacement text: abcdef_xyz123

output: the name of my application is abcdef_xyz123. its my first project