If you have a hardware keyboard connected, you should be able to use “Ctrl+w” to return to the Volumio UI.
If you are using matchbox-keyboard as a virtual keyboard, on the shop page look for “Product categories” and touch “Select a category” below it. This will open a list of … product categories and a search bar on top of them. When you tap into the search bar the virtual keyboard should appear and you can enter “Ctrl+w” from there. Alternatively you can scroll pretty much to the bottom of the shop page. Below the header “Listen to Music the Right Way” there is an input field with a button labeled “SIGN UP” next to it. Touching that input field will also open the virtual keyboard and you can type “Ctrl+w”.