How to restart a pre-order?

Not knowing what you've tried, or what specifically prompts the question other than a genuine gap in knowledge, it should simply be a process of repeating the same steps as you went thru when you originally enabled pre-order for that app.

From the homepage of App Store Connect, go to “My Apps” and select the app. Then, select Pricing and Availability and you'll be able to see the Pre-Orders section (if you have never published that app on the App Store). There, select “Make available for pre-order” and choose a date to release your app for download. Done.

If that's not working, and you have further questions, be sure to provide further details...what you did, what you expected, how those differed.

Good luck.