Go to the Ambassadors tab and open the ambassador you want to change.
On their detail page, click Update next to Promo code, type the new code, and save.
You can point them at a brand-new code or an existing one. Codes have to be unique, so if the one you want is already claimed you'll need to pick a different code.
Let your ambassador know once you've changed it, so they can update the code in their posts and links.