Tap the product and then change the number of items on the detail screen. Depending on the product, you may be able to change the unit on the right-hand side next to the "plus" icon. On our website, you can do this by clicking the three dots.
In the Migros App, you can also add the amount and unit in front of the item name when adding general products like milk or bread. The Migros App automatically recognises the amount and the unit. For example, enter "100 g chocolate".