There are two ways to tell if a presentation is a read-only file. The first is if you see the words Read-Only after the title of the presentation. The second is if you see a yellow message bar across the top of a presentation with a message letting you know the document has limited access.
Method 1
1. Next to the yellow banner message will be a button that reads “Edit Anyway.” Click that button to unlock the PowerPoint.
The yellow bar should disappear along with the “Read-Only” text that appeared after the presentation title. Your PowerPoint will now be editable.
Method 2
1. Open the read-only presentation and select the File tab.
2. You’ll be moved in the Info menu. Here, you’ll see the Protect Presentation option is marked in yellow and has a message displayed next to it—the presentation has been marked as final to discourage any editing. Go ahead and select Protect Presentation.
3. From the submenu, select Mark as Final. Clicking this will undo the lock.
You’ll now see that the yellow banner and its accompanying message next to the Protect Presentation option disappears. The Read-Only text next to the presentation title also disappears.
Information sourced from: https://www.howtogeek.com/437502/how-to-unlock-read-only-powerpoint-presentations/