PowerPoint Online can open files stored on OneDrive, SharePoint, or Dropbox.
Upload a 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.
Upload a file to SharePoint
Sign in to your Office 365 work or school account (.com, .org, .edu) and go to SharePoint.
Navigate to the destination folder.
Select Upload to save the files to SharePoint.
Edit a file with a Microsoft account
When you open a presentation from OneDrive directly or through a shared link with a Microsoft account, PowerPoint Online displays the file in Reading view.
To edit a file:
In your web browser, navigate to the file in OneDrive.
Click the file name once to open the file in PowerPoint Online.
select Edit Presentation, and then select Edit in PowerPoint Online.
The file switches from Reading view to Editing view, where you can:
Add or change content
Share the file and collaborate with others
View the presentation in Slide Show
Edit a file with an Office 365 work or school account
When you open a presentation from OneDrive for Business or SharePoint directly or through a shared link with a Microsoft work or school account, PowerPoint Online displays the file in Editing view by default.