Customize menu > Preferences > Preferences Settings dialog > GCS panel > Global Coordinate System group > Define > Click Define or Modify. > Geodetic > Define > Datum Manager > Modify
The Modify Datum dialog enables you to modify a selected datum. If you want to modify a predefined datum, you must assign it a new code before you can save it. You should also give it a unique description, because only the description is displayed in the manager dialog.
Using this dialog requires advanced mapping knowledge.
Code—Specifies a code. The code must be unique within a category and across all coordinate systems, must comprise of 23 characters or less, and must not contain spaces. Only the following characters are valid: A-Z (upper or lower case), 0-9, dash, and underscore.
Description—Specifies a description. This description, not the code, appears in all list boxes, so be sure it is specific and descriptive.
Ellipsoid—Specifies the currently selected ellipsoid.
Select—Displays the Select Ellipsoid dialog, where you can select a datum for the coordinate system.
Define—Displays the Ellipsoid Manager dialog, where you can create a new ellipsoid or modify an existing one.
Datum Conversion Technique—Lists the available datum conversion techniques. The conversion technique that you select determines which options are available in the Conversion Parameters group.
Specifies the datum conversion parameters, depending on which conversion technique you selected.
Vector Component Deltas to WGS-84 in Meters—Specifies the offset from WGS-84.
Rotation Angle from WGS-84 in Arc Seconds—Enter information in these four boxes if you are using the Bursa/Wolfe or Swiss datum conversion instead of the Molodensky Transformation method.
OK—Closes the dialog and saves any changes you’ve made.
Cancel—Closes the dialog without saving any changes you have made.