Click the configured Start Time (or) End Time Date links
Select Calendar icon 
and reschedule date & time
Click on hover tick mark 
to save it
Downtime Schedule configured in change planning stage will be available under change implementation stage as well, so that the project/task scheduling can be done based on the configured downtime. Apart from this, due to some unanticipated reasons, if there arises a need to reconfigure downtime, the same can be done from change implementation stage.
That is, the configured downtime can be edited from change implementation stage, if required.
Editing Downtime Schedule:
Click the configured Start Time (or) End Time Date links
Select Calendar icon and reschedule date & time
Click on hover tick mark to save it