When you have added all your items to the cart, you realized that you left out one! How do you add more variants without leaving your cart? You can use the "Quick Add Product" feature to add products to a new order on the Cart page.
In your cart, tap the "Quick Add Product" button. A list of all your variants will appear.
Search for the variant (by SKU/Product name/Variant name/Options), select it and add your desired quantity and discount.
Your items will be added straight to your cart.