@Chris Depending on what you want to do you can use hooks like @a_5mith suggested. The hook you'd want to use is: filter:header.build
What I don't like about the hook though is it just appends the new items to the end of the existing ones...I don't think you can change the order, or remove existing ones. I could be wrong though.
-- or --
The menu.tpl for lavender is actually being pulled in from the vanilla theme:
So, just copy the
menu.tpl from vanilla to your lavender partials folder and change as needed.