If you are not in a PGP Universal managed setting, you can generate a new key from the File menu of PGP Desktop (New PGP Key). If you no longer need to be able able to sign with your currently existing key, you can right click on it and choose Revoke. When you revoke a key, it can still be used to decrypt files encrypted to it, and can verify signatures made by it, but you can no longer either encrypt to it, or sign with it.