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
xdg-mime default your-program.desktop \
(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.