Currently clients can only request appointment changes. We are working on a way to allow client to get back to the scheduler so that clients can reschedule themselves based on your team's availability and will update this article once that feature has released. 

The first step for your client to reschedule an appointment they need to navigate to the project they wish to reschedule. Then they need to click on the Order tab on the top navigation bar. 

From here they can click on the pencil icon on the assigned task they would like to reschedule which will bring up a modal window. 

Then click on the "Date and Time" field and a calendar will pop up. Simply choose a date and set a time for the desired appointment and click Request Appointment. 

An email will be sent out to the email stored in the Contact Information in your Account Settings in Setup item on the side navigation bar requesting the change be confirmed.