simonchristieis (is correct, you need to open the PNG; not the html file, but ... there is a better way:
You can open your original PNG file (before the export) and then edit it, and then choose FILE > UPDATE HTML. It should give you options of updating images + HTML or just html.
I *LOVE* this option. We never use this on finished sites, unless we need to change an entire menu or something like that; but this is GREAT for prototyping layouts with quick changes. How many hours has this saved me ... I have no idea, but it's lots!
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I am woman, watch me code.