Once you click on the Text tool, you will see a Styling icon on the right side of the toolbar.
Two ways to save a font as default
Option 1: Setting a default font from the start
Tap the Text tool > Styling icon, and adjust the font style to your liking and tap on Save as Default.
Option 2: Changing the existing default font
Tap a text box with the Text tool, long-press and tap Select All and the text will be selected as pictured. Then tap the Styling icon to change the font style to your liking. Tap Save as Default.
Changing a font temporarily
The default font style you created is used for creating new text boxes. To change the font style temporarily, you can select the whole sentence or parts of it and make adjustments without saving the new style as default.
* Please note: If you want to copy an existing text box, use the Lasso tool 
to make the copy, then long-press and choose Paste.