title: How to change only the subject(CN) in existing csr
body: <p>I have a csr(Certificate Signing Request).</p>
  
  <p>I have to just change the CN from that csr, leaving other fields intact. It is like updating the existing csr.</p>
  
  <p>This should be done automatically. 
  Is there any method to do this in c/c++/openssl?</p>