I have this problem as well. When you buy an iPad Air you get a version of Numbers that might be different than the ones being discussed here. Anyway, I can't figure out how to delete a spreadsheet in my version of Numbers. None of the suggestions here work, at least not obviously. Eduardo -- When I open the sheet I want to delete (is that what you mean by 'select'?) and tap on it I do get a menu and delete is one of those options but when I tap it all that is deleted is the cell immediately under the tap. I can also delete rows and columns by tapping them and choosing the right option, but not the entire sheet. Similarly, Tapajo5, there is no trash icon on the Numbers page in my version, so I have nowhere obvious to drag the sheet to. Not that I seem to be able to drag these sheets anyway. When I try to give a prolonged press the way you do when you want to delete an app nothing happens. Yet it seems impossible that even a beta would be released with no delete option.
Still, it's not physically impossible...