In order to change Apache port from 80 to 81, you will have the change the Apache Configuration. You can do that from the AMPPS Control Center (as from the Image attached below).
We do not recommend in doing this as all the scripts installed via AMPPS is done on port 80. If you change the Apache port from 80 to 81, you will have to manually change the settings of all the scripts installed via AMPPS.
Also you will have to access AMPPS End-User page as /localhost:81/ampps
Please let us know if you need any further information.