In the details window of the desired requirement, the review date can be edited by an owner in the Review Due field. Click on the date field, and select new date from the calendar pop-up. Alternatively, this can also be done in the Reviewers tab. Click on the current requested date and select a new date from the calendar pop-up.
This date can still be changed after a requirement is sent for review. If a date is not appearing, there may be no previously selected day or there are no reviewers added.
Users will need to be the requirement owner or co-owner, and have project permissions to view and edit the requirement to perform this action.