To start with you could use the following code. For security you could also add some nonce check and even use the settings api. Here `plugin-test` is your plugin folder name.

    $file = stripslashes('plugin-test/style.css');
    $plugin_files = get_plugin_files($file);
    $file = validate_file_to_edit($file, $plugin_files);
    $real_file = WP_PLUGIN_DIR . '/' . $file;

    if( isset($_POST['plugin_test_settings']['newcontent']) ) {
        $newcontent = stripslashes($_POST['plugin_test_settings']['newcontent']);
        if ( is_writeable($real_file) ) {
                $f = fopen($real_file, 'w+');
                fwrite($f, $newcontent);
                fclose($f);
        }
    }

    $content = file_get_contents( $real_file );

    $content = esc_textarea( $content ); ?>
    <table class="form-table">
        <tbody>
            <tr valign="top">
                <td>
                    <textarea cols="70" rows="25" name="plugin_test_settings[newcontent]" id="newcontent" tabindex="1"><?php echo $content ?></textarea>
                </td>
            </tr>
        </tbody>
    </table>