This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
YOU CAN'T, since the pass phrase is required to create the
The way to avoid this dilemma is to create a key revocation
certificate at the same time that you generate your key pair. Put the
revocation certificate away in a safe place and you will have it
available should the need arise. You need to be careful how you do
this, however, or you will end up revoking the key pair that you just
generated, and a revocation can't be reversed.
To do this, extract your public key to an ASCII file (using the "-kxa"
option) after you have generated your key pair. Next, create a key
revocation certificate and extract the revoked key to another ASCII
file using the -kxa option again. Finally, delete the revoked key from
your public key ring using the - kr option and put your non-revoked
version back in the ring using the -ka option. Save the revocation
certificate on a floppy so that you don't lose it if you crash your
hard disk sometime.