You can edit message formats in the SMS Control Centre. From the Appointment Book, click Setup > SMS Control Centre

The message format section allows you to set up various messages to be transmitted to patients. These messages use codes (e.g. <patientfirstname>), which will be replaced by relevant data when sent. 

Click on Key to view a list of all available codes for you to use. 

Whatever is left on screen when you press OK will become your default message format.