My name is Ollie, I am part of the Social Care team at Shopify, happy to help out where I can.
Thanks for including the link to the online store. You should be able to make customization changes to the online store by navigating to Online Store > Themes > Customize. This will bring you to the Theme Editor where you can start to make edits to the theme.
Additionally, you can make changes to the coding of the theme by selecting Online Store > Themes > More Actions > Edit code. This should then bring you directly to the files of the online store theme.
Let me know if you have any further questions.