How do I make my company slide deck my default slide deck in PowerPoint?

There are two methods to do this.

The first is to open the presentation or template, then choose File>Save As. Change the Save as type drop down to PowerPoint Template (*.potx). Change the file name to blank.potx, then click on the Save button.

The second is similar. Open the presentation or template, choose File>Save As. Change the Save as type drop down to PowerPoint Template (*.potx). Click in the address field of the dialog, paste in this address:

%appdata%\Microsoft\Templates\Document Themes

then press Enter. Change the file name to Default Theme.potx, then click on the Save button.

Open PowerPoint and press Ctrl + N to test.