How to change your gpg (private) key passphrase
0. If you dont have gpg installed, you can get GPGTools a nifty application suite to manipulate your keys and much more from http://gpgtools.org
1. First get the key_id, by
gpg -K
You will see something like
sec 2048R/EN78SK32 2011-02-19
uid Joe Schmo <joe.schmo@somewhere.com>
You are looking for the line beginning with "sec" and then you can locate the id, in this case it is EN78SK32
Just for reference the 2048R indicates it is a 2048 bit RSA key.
2. Now you can edit the key by
gpg --edit-key key_id
i.e. for our example
gpg --edit-key EN78SK32
3. At the prompt type "passwd" i.e.
gpg> passwd
And you will be prompted to enter your old password first and then enter and confirm your new password.
4. Now when you quit make sure to save your changes:
gpg> quit
Save changes? (y/N) y