The key message here is to never allow a change that is not controlled to creep into your project. Changes will come, that is a natural process. But you need to make sure the change is needed, all implications are understood and the change is documented. You need to make sure that your stakeholders understand your change.
Changes can come from different sources and you can not simply ignore them. Could be that the goals of the organization do not match anymore with the goal of the project. Other times something was change in the environment of the project. You can even have changes generated from inside the project in case something was forgotten or if someone has a better idea how to do things.
Regardless of origin of the change, you as the project manager has to do a few steps every time a change occurred:
You need to understand the change, the reason the change was generated and what are the benefits. You also must understand what is the impact. At minimum you need to see is the impact on the time, budget and scope. This is the minimum and for significant changes you need to see the impact on all project area, including human resources, communication, procurement, quality and risk management.
Only after the change is completely understood, you need to decide if the change will be included in the project or rejected. If the change is approved, it will be part of the project
Make sure that al changes are documented (even the rejected changes) and ensure that relevant stakeholders are informed.
As a general recommendation, change should be avoided as much as possible, as they are a potential risk to the project success. Whenever the change can not be avoided, it needs to be included in the proper way into the project, to keep the risk as low as possible