Customize menu > Preferences > Preferences Settings dialog > GCS panel > Global Coordinate System group > Define > Define or Modify > Geodetic > Define

Datums—Lists the descriptions of existing datums.
Define—Displays the Define Datum dialog, where you can define a new datum. Using this options requires advanced mapping knowledge.
Modify—Displays the Modify Datum dialog, where you can modify the selected datum.
Remove—Deletes the selected Datum from the Datums list.