Picking a font is helping creating a brand for you, it's very important for your business and your store.
To pick a custom font:
tap the text you want to edit
tap fonts
go to the end and tap "Show all"
Search the font you want to use in the list
Here is the video of the demo.
If you can't find the font you want in this list, an option is to copy and paste your text directly inside the text area in the app.