From where a change request raise ?
Changes may be requested by any stakeholder involved with the project (PMBOK6, pg 115)
Is it only when there change to the project scope and plan or it could happen when there are a defects on the deliverable ?
As a general rule, each project's configuration management plan should define which project artifacts need to be placed under configuration control. Any change in a configuration element should be formally controlled and will require a change request. (PMBOK6, pg 115)
What is the steps to make a change request ?
I don't think you need to know the step on how to submit a change request for the exam. However, it is important to know the process of handling a change request:
1.Evaluate the impact
2. Identify options
3. Get the change request approved internally
4. Get customer buy-in if required
(Rita v9, pg 161)