Zooming out in JOSM slippy map is not very well documented, there are currently three duplicate bug reports about it.
When you press the download button of JOSM to download OSM, you are presented a slippy map like on the OpenStreetMap homepage. It is pretty easy to zoom in: Just double click. But how do you zoom out? If you have a mousewheel (which requires a mouse), you can use this to zoom in and out.
If you look into the source code, you find another way: Hold Alt and press up/down arrow. Or hold shift and up/down. Or hold control and press up/down. Or press the "plus" and "minus" keys of the keypad. If you work with a laptop, you probably have to press a "Fn" or similar key before.
