Scheduling publishes the draft exactly as it stands when the time arrives. If you keep editing after scheduling, what goes live is whatever the draft looks like at that moment, not what it looked like when you set the time.
The time you pick is your own local time, so 9am means 9am where you are. If you change your mind, open the same panel and cancel.
Step by step
Make your changes in the editor as normal.
Press the schedule button beside Publish.
Pick a date and time.
Leave it. You do not need the editor open, or your computer on.
If it does not go to plan
Can I schedule more than one change?
One scheduled publish per page at a time. Setting a new one replaces the old.
It did not go live.
The commonest cause is billing: a site that cannot publish cannot publish on a schedule either. Check the plan card, then tell us.