Go to the Tickets tab -> click on a ticket, in the Ticket details page, you'll find the 'Edit' option right above the Subject of the ticket.
1) The option to change the due date will only show up when the ticket is assigned to statuses that have the SLA timer ON (example: Open).
You can check which statuses have their SLA timers ON or OFF under Admin > Ticket Fields > Status field. Once a manual change is done to the Due Time of a ticket, it will not change again when the ticket properties (for example, a change in priority) are updated.
2) The due by date and time can always be updated only to a value greater than the First response time, that is the 'Response due' time on the ticket.