Management of the DDI's is generally better done via the DDI summary where possible.
Point to note, if a DDI has been directly assigned anywhere outside of DDI summary either to a user or a group etc then you will not be able to make a change to it in DDI summary and it will have to be unassigned first before alterations can be made.

Changing DDI destination in DDI Summary

Menu Location: Call Manager / Call Center / DDI Summary

Find the required DDI via the search dropdowns - Site (choose site) / DDI (enter ddi) / Convert Digit (enter ddi destination)

Highlight the line for the required DDI and click Modify:

Choose the Time setting for the DDI - Time Schedule / Always

The Always option sets a single permanent destination for the DDI.  If the DDI is already assigned directly to a user, group etc. then this will already be populated and you will not be able to change it without removing it from it's assignment first. It will clearly show in the DDI list if there is a destination already assigned.

Image (ddi_summary_assigned_destination)

The Time Schedule option will give you entry boxes for Day / Night / Timed destinations.  These work using the time schedule for the site the DDI is assigned to.

Image (ddi_summary_time_schedule_destinations)

Click save to apply any changes