Your Zip order installments will be charged to your default payment card on their set dates automatically. If you'd like to pay an installment early, or need to pay off an outstanding installment using the Zip app, go to the Orders tab.
Then, select the order you want to make a payment for, and tap Pay on the installment. You are welcome to pay all your installments early, but each installment must be paid one-at-a-time.