Open the PowerPoint presentation you want to keep as the main file
Go to the slide where you want to insert the other presentation
Click the Home tab
Select New Slide
Choose Reuse Slides
Click Browse
Select the PowerPoint file you want to merge
Browse through the slides in the reuse pane
Click each slide you want to add, or click Insert All Slides
Check Keep source formatting if you want to preserve the original design
Rearrange the slides as needed in the thumbnail pane
Save the merged presentation with a new file name
Alternatively, open one presentation and copy slides from another
In the source presentation, select the slides you want
Right-click and choose Copy
Switch to the destination presentation
Right-click in the thumbnail pane and choose Paste
Choose a paste option if prompted
Save the combined file