By definition change request is a procedure to edit the scope of a project.
Sounds simple but by experience its one of the most challenging tasks for a project manager. Change request could be due to a variety of reasons its not just because of the lack of clarity in scope or someone missing our a requirement in the scope document. It could be well due to market conditions, change in technology etc. A change request could have a deep impact on your project in terms of scope, time, cost etc and could lead it to a failure.