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There is no menu option for the default powerpoint program in Details > Default Applications in Debian Linux, in Fig. 1.
A terminal command and/or GUI tool is welcome for the task.

Testing Stephen's oneliner

Commands and their outputs

$ xdg-mime query filetype test.ppt
application/wps-office.ppt

$ xdg-mime query default $(xdg-mime query filetype test.ppt)
wps-office-wpp.desktop

Testing setting up by the outptus the default ppt viewer

xdg-mime default wps-office-wpp.desktop 
application/wps-office.ppt 

Output: no effect in opening .ppt files; other lines are not needed because VND and others are not in my system

OS: Debian 9.1 Linux

Powerpoint: WPS Presentation, currently LibreOffice presentation as default

There is no such thing as a “default PowerPoint”; you need to define the default handler for the various MIME types associated with presentations. To do so, determine which .desktop file represents the program you wish to use, then (as your own user, not root) run

xdg-mime default your-program.desktop 
 application/vnd.oasis.opendocument.presentation 
 application/vnd.oasis.opendocument.presentation-template 
 application/vnd.sun.xml.impress 
 application/vnd.sun.xml.impress.template 
 application/mspowerpoint 
 application/vnd.ms-powerpoint 
 application/vnd.openxmlformats-officedocument.presentationml.presentation 
 application/vnd.ms-powerpoint.presentation.macroenabled.12 
 application/vnd.openxmlformats-officedocument.presentationml.template 
 application/vnd.ms-powerpoint.template.macroenabled.12 
 application/vnd.openxmlformats-officedocument.presentationml.slide 
 application/vnd.openxmlformats-officedocument.presentationml.slideshow 
 application/vnd.ms-powerpoint.slideshow.macroEnabled.12 
 application/vnd.oasis.opendocument.presentation-flat-xml 
 application/x-iwork-keynote-sffkey

(filtering the list as appropriate). This will use your-program to open OpenDocument presentations, OpenOffice/LibreOffice presentations, PowerPoint presentations, and Keynote presentations.

You might need to add other MIME types; to determine which ones to use, run xdg-mime query filetype your-file, replacing your-file with the name of a file you want to configure. This will give you the MIME type to add to the list above.

If it’s correctly written, your chosen application’s .desktop file should have a MIME type entry listing the types it supports — you can use that instead of the above list.

Alternatively, since you’re using GNOME, you can right-click on your file in the file manager, select “Properties”, then the “Open With” tab, and choose the default application there. You’ll need to repeat that for all the different file types you want to open with WPS Presentation.