To edit a presentation in PowerPoint Online, you must store the file in OneDrive or Dropbox, and it must be in .pptx or .odp format.
Save the file to OneDrive
If your file is saved on a computer hard disk, a flash drive, or a USB stick, the first thing to do is upload it to OneDrive.
Open the presentation for editing
When you open a presentation from OneDrive with a Microsoft account, PowerPoint Online displays the file in Reading view.
To edit the file, select Edit Presentation, and then select Edit in PowerPoint Online.
Once the file is open in Editing view, you can add and delete content (such as text, pictures, and tables) and do other things, such as:
Share the file and collaborate with others
View the presentation in Slide Show
PowerPoint doesn't open Google Slides files
PowerPoint Online doesn't open Google Slides (.gslides) presentations. It only opens .pptx files or .odp files.