To edit a part of the primer first select the section you want to work on. Then check that it is not locked. There is a little lock icon next to the document. Request to edit it and set the "Lock This Document" tick box and update the document.

Now check that you got the lock rather than someone else and proceed to edit the section you want to work with.

When you are done, go edit the document replacing the current version with your updated version and un-tick the lock. Update the document.

I recommend using chang tracking.