How do you change the Export DPI of Microsoft PowerPoint?
Assuming I understand your question correctly, you can change the DPI by altering the width and height upon export.

For example, if you want to export your slides in PNG format, you would:


Go to File > Export...
In the Export window that appears, select the PNG option from the File Format drop-down list (at bottom centre of window)
Now you will see options for Width and Height (see image below):





Now change the value of either option (this will automatically change the other value to ensure you keep the same ratio)


By changing the dimensions, you also change the DPI. For example, a 720x405 image exported from PowerPoint will only be about 54dpi, while doubling this upon export to 1,440x810 will also double the dpi to 108dpi.