It is not possible to readdress a letter once you have saved a referral reply letter. You would need to cancel the letter and start again.
If you have not yet saved your Referral Reply letter, you can choose the Readdress option and search for a new recipient.
If you are using Open> Import Transcribed Letters to import letters created by your typist, you can change the Addressee at the Import screen. Simply double click in the Addressee column for the letter and you will be presented with the Address Book search window. Search for your new recipient and select OK. Once you import the letter, whoever is listed in the Addressee column is who Genie will make the Addressee in the letter.