Any changes made to the specifications of your project that require changes to already-submitted deliverables (in order to keep them correct), require the approval of the TA. To get this, you need to submit an "Engineering Change Proposal" (ECP). If this is approved, you need to add modified versions of all your documents (preferably with a change section in the new document mentioning what changed, along with the changes in their normal locations) to your project notebook. However, it's important that you keep working (as if the change wasn't even requested) while you wait for approval. Don't stop work just to wait for a change to be approved.
The ECP consists of a detailed plan of all of the changes requested. It must describe what changes need to be made to every deliverable submitted so far, at a level that allows you to understand how much it will change your time and schedule commitments. You also need to include a "purpose" section, describing exactly why you want to make the change you're proposing. This may be useful farther down the line, when you want to see how well you've met your goals.
The TA and possibly the professor will review your proposal, and decide whether the proposal is worthwhile, warning if they think it will require too much effort.
Summary: A paragraph or so describing the outline of what you plan to change.
Purpose: A few sentences describing why you propose this change
Documents affected:
The number of changes in each case is, of course, dependent on what exactly you plan to do. Along with this proposal, you should include modified versions of every document affected by this change, as described in the proposal. Modified documents should also include a change log section, describing when and how they changed.