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 Customization > General > Fields > [Object Class].
Change the field label and other parameters as needed.
Click Save on the toolbar.