Once one field is 'active' for editing, pressing tab should cycle through all the other fields. You can 'activate' a field either by clicking on it, or by tabbing to the metadata panel. That last one is a bit subtle, basically, press tab repeatedly and you should notice how the keyboard focus cycles through the search field, the sidebar, the item table/grid and the panels (when a panel gets the focus, the background changes subtly). When the metadata panel has the focus you can hit the 'Arrow Down' or 'Enter' key to descend into the first field: once the field is activated pressing tab should cycle through the fields (*) -- does that not work for you? If it does not, please let me know which operating system you're on so that I can try to reproduce. Thanks!
(*) I just noticed that Shift-Tab, to go backwards through the fields takes you back to the item table; that's actually a bug, it should take you back to the previous field.