The overall duration and start date of a project schedule is based upon the subtasks of the schedule.

So if you want to change overall duration and start date, you will edit the durations and start dates of the subtasks in the project.

Here's a quick video for clarification:

‍

Note: if you have Auto Scheduling turned on, a task set to Automatic can still be rescheduled based on its dependencies, so a start date you enter may shift when an earlier task changes. If you want a task to hold a specific start date, set that task to Locked in the "Auto?" column. A locked task keeps the date you set, while the rest of your schedule keeps rescheduling around it. See "How do I lock a task to a specific date?" for the steps.