For Google Slides users:
Click on File, go to Download as, choose Microsoft PowerPoint (pptx) and you are done!
For PowerPoint users:
You have to log in to your Google Drive account, click New, select File Upload and upload your PowerPoint presentation from your computer.
Once it is uploaded, right click it and select Open with, and choose Google Slides.
This will create a copy of the PowerPoint file in Google Slides format.