It should do it all automatically,
I seem to remember on long trips it would periodically recalculate my route when its data suggested roads that would have been congested an hour or two ago are now free flowing enough to use. But only if it is of benefit and / or reduce the overall time of the journey.
I think it constantly monitors the time of day and the data it has for that time period.
I guess one way to check this would be to set a long route passing through a number of cities, put it by a window for a gps fix and leave it for a few hours to see if the route changes over time.
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