A to-do is displayed as upcoming overdue based on the due date that has been assigned to it and your personal settings.
Upcoming
By default, to-dos will be listed as upcoming if the due date is within 5 days of the current date.
You can update this window by going to Settings > Personal. Adjust your preference for "To-do items & schedule tasks upcoming within ___ days" and be sure to save your changes. Changing this value will only affect you, and not anyone else on your team.
When an item is upcoming, you will see a yellow "upcoming" indicator on the to-dos page and in your account's Task Manager.
Overdue
If a to-do has a due date, the item will be marked overdue if the to-do has not been completed by that date.
Items without a due date will also be shown as overdue if they have been left open for too long, as determined by your personal settings.
By default, items without a due date will be shown as overdue after 7 days have passed.
You can update this window by going to Settings > Personal. Adjust your preference for "To-do items overdue in ___ days if no 'Finish by' date set," and be sure to save your changes. Changing this value will only affect you, and not anyone else on your team.
When a to-do is overdue, you will see a red indicator on the to-dos page and in your account's Task Manager. It will also be visible on the main dashboard for the assigned users.