1. First of all, the simplest thing is to directly click on the font on the document to select and change it to the desired font.
2. If you want to permanently modify the default value, click the three-point option in the upper right corner.
3. Select Settings.
4. Change the font or font size without saving.
You can find that the default font has changed, but the original document will not be affected by this. If you want to change it, you can only do it manually.