The cn=schema entry has a multivalued attribute, attributeTypes, that contains definitions of each attribute type in the directory schema. You can delete definitions with X-ORIGIN 'user defined' by using the ldapmodify(1) command.
Because the schema is defined by the LDAP view in cn=schema, you can view and modify the schema online using the ldapsearch and ldapmodify utilities. However, you can delete only schema elements that have the value ’user defined’ for the X-ORIGIN field. The server will not delete other definitions.
Changes that you make to user-defined attributes are saved in the file 99user.ldif.