overwrite a single line of text

I'm having problems overwriting a single line of text. If the line I'm replacing is shorter then the new line. The subsequent line gets truncated into the line above. Note example below (</head> tag)? I've been able to fix it, if the new line is shorter, i keep adding spaces. (not ideal fix)! Why do I have to do this, isn't there a way to rewrite the entire line up to the "\r\n"? ------ <html> <head> <title>Fix the damn css link</title> <link rel="stylesheet" type="text/css" href="basic.css">/head> <body> <h1>css link</h1> <hr> <p>something goes here. <br>