Customization in data model

If you are asking about the database data model, then it's simple: you cannot change it.

If you are asking about the conceptual data model, in other words, how Windchill/Integrity is used by end-users, then it's an enormous subject: how to create/modify fields, item types, workflows, triggers, permissions, queries, etc., etc.

You need a lot of training for that and there is almost no usable documentation. I suggest you talk to the system administrators in your organization, if you have any, or PTC Services.