[GeoffR]

The epub reader has only a very small top margin built-in, in firmware 3.2.0 it is just 15 pixels on the Glo, or less than 2mm. So, if you are reading kepubs, then one way to reduce the top margin is to sideload the book as an epub.

If you are already reading epubs and your top margin is larger than 2mm, then you can edit the book to remove the @page margin/padding. But if you want to remove the remaining 2mm top margin from the built-in epub reader, then I don't think anyone knows how to do that yet.

Edit: Attached a screenshot from my Glo showing a page from an epub with no extra top margin/padding in the book, just the 15 pixel top margin built in to the epub reader.