To edit the date of a task you can either use the quick edit option or open the task to the detailed view and edit the date there.
Quick edit
On the task board, identify the task you want to edit the due date for.
Click on the 'quick edit' icon on the task.
You'll then have the option to edit the due date of the task.
Open the task and edit the due date
The other option is to open the task to it's detailed view by clicking on the task name.
Then scroll to the bottom and you can edit the due date in the 'delivery' box.
When Hivelight won't let you select a later due date for the task
The due date for a task can not be later than the target date of the milestone it belongs to. Were it otherwise, you could complete all the tasks on time and overshoot the ultimate deadline that the tasks relate to (the milestone).
To select a later date for the task due date, you'll need to edit the milestone target date (instructions here) which can be done in a matter of seconds so long as you have the permission level to do so (learn more here).