Open, remove text, and save a text file C# : I want to edit some text files automatically but I don't know what to used to do it.

I want to open a file, check all lines, if one line begins with a 'C', I remove first 39-characters, etc .. then save all the file with an other name.

I already have a portion of code : 

    var car = ligne[0];
    if(car != 'C') { continue; }
    ligne.Remove(0, 39);

I use StreamReader to read, but what is the simple way to read and save in another file ?