Launch your DealMachine mobile app or log in with your computer to https://dealmachine.com
Click the menu button in the top left (mobile only)
Select the signature you wish to edit. (You only get one signature with the Basic Plan)
Check the "Signature Sign-off" and change it to say "Sincerely", "All the Best" Etc.
Save your changes and your information will show up on every template that includes this signature.
After doing this, you may still need to go inside a deal and hit "Reload Preview" below the mail preview to see the updated information on the postcard.