It depends on how significant the change is and whether it relates to a course or a module. A minor change, such as updating the bibliography/booklist for a module, does not require formal approval (but please tell the Library!). A major change, such as changing the assessment methods for a module, will normally need approval.
For changes to courses, proposals will need to be submitted to the same committee which approved new things - for such course changes it's the Faculty Graduate Studies Committee, Undergraduate Studies Committee or Sub-Faculty, and then the Board of Graduate or Undergraduate Studies, as appropriate, as well. There is something of a grey area between obviously minor changes and obviously major changes, where it's best to check with the secretary and/or chair of your Faculty committee.
Major changes to modules will need to be approved by the relevant departmental committee which also approves new module proposals. Further guidance on what constitutes a major change is given in the Guidance for Academic Departments on Module Approval