To change the passphrase of your key identity file just type:
ssh-keygen -pFirst you will be asked for the location of the file. It will suggest a default and that should be the correct answer if you didn't change the default when creating the key. So you should just be able to hit <ENTER>.
Second you will be asked for your old passphrase, followed by a request for your new passphrase, twice.
(information coming soon...)
(information coming soon...)