Yes you can cancel anytime, there is no contract unless otherwise signed up with one.
When you cancel, you will not be charged from the next month onwards. 

Yes you can.

Optopro provides both options for document numbering, you can have same series throughout all stores or have individual serise per each store.

Same socument series throughout all stores

1. Navigate to Administration -> System Settings -> Document Prefixes
2. Specify the Document Prefix for the required module as INV-{0:D6} where INV is a fixed text (as per your need) and {0:D6} is dynamically generated 6 digits number in sequence.

Unique Document series for each store

1. Navigate to Administration -> System Settings -> Document Prefixes
2. Specify the Document Prefix for the required module a INV-{1}-{0:D6} where INV is a fixed text (as per your need) and {0:D6} is dynamically generated 6 digits number in sequence per sote and the {1} is the prefix for the store. The prefix for the store can be defined in Administration -> Company Settings -> Store

Yes you can. 

To setup your Gmail account to send out emails from Optopro, you need to App password in you Google Account. Once you set it up, google will give you an 16 character password. This 16 character password should be used in Optopro along with your Email address to start sending out emails from your Gmail account.

You can visit the following link to learn more setting App password:
Sign in with app passwords – Google Account Help and click https://myaccount.google.com/apppasswords to set App password.

Email Server: smtp:gmail.com
Email Port: 587
SSL Enable: Yes
Email Name: YOUR NAME
Email: YOUREMAIL@GMAIL.COM
Passwrod: YOUR 16 DIGIT APP PASSWORD

To reschedule your current appointment, please follow these steps: 

1. Select the appointment from the calendar
2. Right-click on it and select "Edit" 
3. Choose new date and time 
4. Save

You have successfully updated your appointment.