modify rsa’s passphrase

1.know old passphrease.
2.ssh-keygen -p
Enter file in which the key is (.../.ssh/id_rsa): github_rsa
Enter old passphrase: if not set the passphrase before, no this line.
Enter new passphrase (empty for no passphrase) :
Enter same passphrase again:
Your identification has been saved with the new passphrase.
3.modify finished.