You can either drag and drop items, or use the ALT + SHIFT + Arrow keyboard combination to move items.
Moving is now even easier with the latest improvements to the drag and drop feature when using Workflowy from the web browser or desktop app. Simply click and drag a bullets (or highlighted selection of multiple bullets) to another position on the screen. You can move the item below other bullets, or below and then slightly to the right to indent the item.
To move items using the mobile apps, just tap on an item (when not in editing mode), then hold and drag to the new position. Please note, it will work a bit differently than described above as you will not be able to choose an indented position when dragging. You will instead need to have your destination tree expanded if you wish to drop the item as a child/sub-bullet.