To enable discounts, go to Settings > General > Allow product-level discount. 

Here's how to give a discount on the product level:

Step 1
​Add the product to the cart 

Step 2
​Tap the product and you’ll now see 3 discount options: 

Step 3
​Once you’ve added your discount you can choose the right product quantity, add a note and save the discount. 

Step 4
​You can now proceed with the payment.