How I can delete a line of UI text

Like my question says, I need to delete a line of a UI Text. I can count lines with preferredHeight but I need delete the first line of my text and this is for read only.

I’m doing a autotyping text and I want that in writing a certain number of letters is delete the first line and continue typing.

The autotyping I already have made, if anyone helps me to remove the first line would appreciate it, as the text appears below every few letters and not by a newline.

Sorry if my English is bad

string text;
int numb; //number of characters you wnat to delete

    int index = text.IndexOf(System.Environment.NewLine);
    index *= -numb;
    var sms = text.Substring(index + System.Environment.NewLine.Length);