How do I change the times of a product on a specific day?

Times belong to the availability rule of the option.

Open your product, then Options (calendar and pricing) and the option. In the Calendar tab, click the slot you want to change: the rule panel opens.

Change the times under Start times. The Add a time button allows several departures on the same day, in 5-minute steps. Click Save: the change applies immediately.

Careful: the change applies to the whole rule, so to every day it covers. To change one date only, create a rule dedicated to that date, or close the existing slot and add another one.