Never ever touch the system tables! They are protected and for a very good reason.
SQL language basically contains two different parts. DML and DDL.
DML[
^] (Data Manipulation Language) is used for modifying the data in user tables. This is what you use when you add, remove or update records in your own tables.
DDL[
^] (Data Definition Language) is used to modify for example the database structure: add tables, columns, create procedure etc.
All the DDL commands do actually use DML to modify the contents of the system tables where the structure is held. But since all of the internals and logic of the system tables are not published, there's no reliable way to directly modify the system tables so that the system wouldn't for example get corrupted.
If you have a specific modification you need to do to the database structure, post it as a new question and I'm confident that you'll get help.