Select the heading text
Open the Styles pane or Home tab
Click the current heading style
Choose Normal or another non-heading style
If Aptos remains, change the font from Aptos to your preferred font
Select all headings you want to change
Use Find and Replace to locate heading styles if needed
Modify the Heading style to use a different font
Right-click the heading style in Styles
Choose Modify
Change the font from Aptos to another font
Click OK
Repeat for each heading level if needed