As of September 5, 2020, Apple can't change third-party fonts, only its own fonts. The replacement method is as follows:

Take Apple 8 as an example. First, click the "Settings" button on the phone desktop.

2. Click the "General" option in the list to enter.

Third, enter the "auxiliary function" next.

4. Click "Subtitle and Close Subtitle" in the "Media" column to enter.

5. Click "Style 1" to enter and "Create a new style".

6. Select the new font you like in the font, return to the style sheet, and check the style 2.