1. Click on the “Overview” button
2. Click on the three dots in the bottom right corner of the page you want to delete. Note that you can select more than one page if you want to delete several at once.
3. Click on the small trashbin in the top right corner of your reMarkable display

There have been reports that the reMarkable won't let you delete a QS page. In that case just repeat step one and two until it works.