I'm not sure what you mean by alter the preset schedule? Are you asking to be able to edit a schedule via voice control? The Google Assistant integration lets you override an existing schedule, but not edit it permanently. "Ok Google, turn off all manual control" will revert all your heating to schedule, or you can specify a particular TRV to return to schedule. I use the former in my Bedtime routine, to ensure any active overrides are cancelled and the heating returns to schedule for the night.