Don't know if the title of your thread is all you're asking, but in any case the answer is that you cancel at any banjercito. You need to know where these are--sometimes at the border, sometimes 20 km before the border. The worst that will happen is that you need to backtrack (in ridiculous heat or even more ridiculous tropical rain) if you miss the banjercito on your way to the actual border.
People post banjercito locations here and on other forums, with elaborate directions and/or coordinates. You can also find the banjercito's own website, part of which is in English. Search for it once you know where you're going to cross into Guatemala or Belize.
Hope that helps.
Mark
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