Basically, you read the file to variable, then fill with value of this variable, and then on form submit update DB value with one from textarea.
Though, you have to watch encoding changes or text can be screwed.
For exact functions, as it is PHP, you can look at http://www.php.net/manual/en/ , it has nice search, examples, and is totally user-friendly.
solved How to display content of txt file?(with the ability to edit) [closed]