If you have items in your cart, a floating cart icon will appear on the app home screen. The icon will not be there if you are working within the checkout process or if there are no items in the cart.
If the app version you are operating is below 5.1, you will not be able to access the cart if you have left checkout mid-process. Your items will still be saved in your cart, so you can still checkout on the desktop site. If this is the case for you, we would highly recommend upgrading your App version if possible.