Working with multiple presentations
To open another presentation, in the File tab, choose Open or click Ctrl+O and select the presentation.
When more than one presentation is open, you can switch among them by selecting the one you want to see from the taskbar in Windows. E.g.:

Alternatively, you can do one of the following:
1. On the View tab, in the Window group, click the Switch Windows button:

Select any open presentation from there:

2. Switch between open windows of all applications by:
- holding down the Alt key and pressing Tab to browse thumbnails of open windows (Alt+Tab) - not just PowerPoint;
- pressing Alt+Esc repeatedly to cycle through them - not just PowerPoint;
- pressing Ctrl+F6 to switch between just PowerPoint presentations.
See also this tip in French: Travailler avec plusieurs présentations.