How does Keats convey the theme of rebirth through death in "To Autumn"?

Autumn itself is the season that precedes the death associated with winter. It begins with abundance, the ripening of the fruit, the grain, and so forth, and it ends after all the crops have been harvested. The reapers come and then the gleaners, and everything is brought in and stored up for use when nature stops giving to us for the year. The poem refers to the "half-reap'd furrow[s]" and the "hook" of the reapers (i.e., their scythes), the coming on of sunset, and the days drawing to a close sooner and sooner each night. Time and/or death are very often represented as a reaper—consider the Grim Reaper.
In the final stanza, the speaker compares the beauty of spring to the beauty of fall, referencing the fall clouds that "bloom" at sunset of the "soft-dying day" and the "stubble-plains" that have all been cleared and readied for winter snows. Even the small gnats are "wailful" and "mourn" the coming changes in season. Thus, the end of fall shows us the death that is near, while the beginning of fall is ripe and heavy with all the fruits of nature's rebirth that begin again in spring.