Once you have chosen delivery vs. pick up and chosen a fulfillment time, the system works much the same as it always has.
You are brought to your cart which now displays the available restaurants on the left hand side.
If you decide you prefer to look back at different times or want to change between pick up and delivery simply click on the grey button on the left side of screen, above the restaurant list.
See screen shot below. The button is called "Change time and reset order".