A passphrase is a word or phrase that protects private key files.
It prevents unauthorized users from encrypting them.
Usually it's just the secret encryption/decryption key used for Ciphers.
To change the passphrase you simply have to read it with the old pass-phrase and write it again, specifying the new pass-phrase.
You can accomplish this with the following commands:
$ openssl rsa -des3 -in myserver.key -out server.key.new
$ mv server.key.new myserver.key
The first time you're asked for a PEM pass-phrase, you should enter the old pass-phrase.
After that, you'll be asked again to enter a pass-phrase - this time, use the new pass-phrase.
If you are asked to verify the pass-phrase, you'll need to enter the new pass-phrase a second time.