Dear Dr. XYZ（This should be the last name of the journal editor who wrote to you.）,
Thank you for giving us the opportunity to revise our manuscript. We would like to request an extension of XXX weeks（Add the number. Do you need 1 week, 2 weeks, 2 months?） for submitting the revised version. To fully address the reviewers’ comments, we will need to carry out new experiments. We estimate that we will need YYY weeks to complete the experiments and another ZZZ weeks to analyze the data and finalize the paper. （Briefly give your reason for the extension. Add the numbers.）
Thank you for your consideration and I look forward to your reply.