To send letters you have previously sent:
Simply access 'Letters' at the top bar.
Select the letters you wish to send again and click the button 'Resend letters'.
Choose or create a template to your new letter.
Edit the recipient address and recipient details, if necessary.
Select the letter line, click 'Add to mailshot' and then click 'Go to next step' at the button right corner.
At the Summary, choose one of the send options, a name for your mailshot and click the green button at the bottom right corner to complete the process.