You are able to send SMS appointment reminders manually as well as automatic reminders. See this article on how to configure automatic appointment reminders.
To send a manual reminder click on the appointment. This will bring up the appointment details screen as shown in the image below.
You should then click on Communication which will bring up a list of the types of communication you can send. Click on SMS and then choose Appointment Reminder. Please note: You can find out how to set up your SMS templates via this article.
This will then bring up the screen below containing your appointment reminder text. You should then click Send.