Follow these steps to change the default font of slide titles in PowerPoint:

1. Step 1: Open the design tab of the ribbon
2. Step 2: Select ‘Slide Size’, then ‘Slide Orientation’
3. Step 3: Select ‘Schemes’, then ‘Fonts’
4. Step 4: Adjust all fonts settings as desired, then select ‘Set as Default’
5. Step 5: Select ‘OK’