Choose Remaining from the menu above the sidebar in the View bar (or in the View ▸ Sidebar Filter submenu); all of your projects that aren't completed or dropped appear.
Choose Next Review from the Grouping menu in the View bar (or in the View ▸ Grouping submenu) to organize your projects by when they are to be reviewed.
For each project in the Review Today group (or earlier groups), check whether the project is in the state it needs to be in: Should you revise its actions, or add new ones? Should you put it on hold, drop it, or complete it? Or is it just fine?
Once you're satisfied with the state of the project, select it and click the Mark Reviewed toolbar button or choose Mark Reviewed from the Edit menu. The project's next review date moves forward and you can go on to review another project.