If the upload button is not showing on your product page, you will need to re-install into your theme, you find the manual install button on the "My Shops" Page inside the app.
1) To Edit Visit "My Shops" page inside the App
2) Find the "Update app for new theme" link
3) Wait up to 2mins for the process to complete
4) Check your product page again (clear cookies/cache and refresh)
if the upload button is still not showing, you will need to change themes inside shopify. First confirm upload button is showing on new theme, if it is, then delete the first theme, and then re-add, before publishing as primary theme again.