<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _update_release_content(self, release, content):
 """Merge existing and new release content."""

        if release.body:
            new_body = []
            current_parser = None
            is_start_line = False
            for parser in self.parsers:
                parser.replaced = False

            # update existing sections
            for line in release.body.splitlines():

                if current_parser:
                    if current_parser._is_end_line(current_parser._process_line(line)):
                        parser_content = current_parser.render()
                        if parser_content:
                            # replace existing section with new content
                            new_body.append(parser_content + "\r\n")
                        current_parser = None

                for parser in self.parsers:
                    if (
                        parser._render_header().strip()
                        == parser._process_line(line).strip()
                    ):
                        parser.replaced = True
                        current_parser = parser
                        is_start_line = True
                        break
                    else:
                        is_start_line = False

                if is_start_line:
                    continue
                if current_parser:
                    continue
                else:
                    # preserve existing sections
                    new_body.append(line.strip())

            # catch section without end line
            if current_parser:
                new_body.append(current_parser.render())

            # add new sections at bottom
            for parser in self.parsers:
                parser_content = parser.render()
                if parser_content and not parser.replaced:
                    new_body.append(parser_content + "\r\n")

            content = u"\r\n".join(new_body)

        return content