You can change the definition of an existing user-defined field except for its type.
TIP: If you want to change the type of a UDF, you can only delete the existing field and create a new UDF of another type with the same name. Note that all data stored in this deleted field will be lost.
To modify the definition a user-defined field:
Go to General > Fields > [Object Class].
Alternatively, you can access UDFs for a particular object class under Workflow and Business Logic (Workflow and Business Logic > [Module] > [Object Class] > Fields) .
TIP: To access UDFs for Activity Log records, go to Miscellaneous > Object Activity (General > Fields > Miscellaneous > Object Activity) .
Double-click the user-defined field. The User Defined Field window appears. The window has two tabs:
The General tab contains all field's properties;
The Usage tab displays the list of workflow items and components that use this UDF, i.e. have placeholders for this field's value.
Click Save on the toolbar.
IMPORTANT: You cannot rename UDFs that are added to the All Tickets / My Tickets grid. For details, see Adding Ticket fields.