[solved] replace char with string UNITY

I see that there is a someString.Replace(char, char) and someString.Replace(string, string).
How can I do this: someString.Replace(char, String) ?

when I just do:

string newString = oldString.Replace('a', 'abc');

Unity thinks I want to replace a char and says: Assets/InputScript.cs(22,72): error CS1012: Too many characters in character literal

Suggestions?
Thanks

Have you tried:

string newString = oldString.Replace("a", "abc");