In the process each user task has it own due date (some by hours and other by day) including the last user task, but they need to set a due date for the whole process in general .

For example: if the process due date set to 3 days, and each user task inside has different due date (hours or days) , if some instance took more than 3 days in general to finish regardless the due dates inside each user tasks, the report should show the instances that reached due date highlighted or in categories and when they click each instance they can see the user tasks and which one was on time or due date and other details. 

The current reports showing the categories (On Time, Overdue & Due within 24 hours) only for tasks (finished or currently running) not on whole instance. 

This is not something we currently support with due date functionality. You could try and do a workaround by adding a branch off the start event with a user task called Due Date and put the process due date on it. The rest of the process would remain the same and if you wanted to check the due date on each user task it would be there, but you could also check the due date on the Due Date user task to see if the instance was completed in time.