I have two x509certificate, and now they are expired. What I would do is to extend the validity date of these certificates (I don't want t creat new one). Can I do this with openssl or other tools ?
No, the only way to change the validity date is to re-issue.
The reason is that the certificate's hash is calculated after the rest of the certificate is written, editing that field would cause the certificate's hash to change. If the hash is changed anyone else checking the certificate will know it has been altered, but won't be able to tell what changed. While this won't automatically prevent a connection, it will throw errors and warnings.