For most ebook readers, tapping and holding on the filename or icon in the files list will pop up a context-sensitive menu. "Category" or "Info" is almost always a possible selection. "Category" will allow a direct change. If not, "Info" will bring up an information dialog that should have a pull-down list arrow next to the category name. Tapping on that arrow brings up the category list. eReader from Palm Digital requires you to actually open the document first, then select the Info icon at the bottom.