In the ticket details page, above the subject of the ticket, there is an option to manually edit the 'Due Date' of the ticket as shown below :
You can also change the SLA applied to this ticket under Admin > Workflows > SLA Policies. Once the changes are done, you can update some properties for that ticket so that the new SLA will get applied and the due by date will be changed accordingly.
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 > Workflows > 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.