On the Order Details page for a past order, a Reorder button appears next to each Realtime-menu or Custom Meal Builder order section (if the business has reordering turned on for that order type) — and you'll also see a smaller Reorder button next to each individual meal/item.
1. Click the section-level Reorder button to pre-check every item of that type, or click Reorder on one specific item to pre-check just that one — a popup opens either way, where you can still check or uncheck items.
2. Pick an upcoming delivery date from the calendar shown in the popup.
3. Click Edit options to tweak your selections first, or Go to Cart to add everything straight to your cart so you can check out.

This button only shows up for order types that support it — you won't see it on every past order.