A downloadable demo of Pokkén Tournament DX was posted at 12am EST on August 24th, 2017 on the Nintendo Switch eShop. Visit the Nintendo eShop on your Nintendo Switch console and find Pokkén Tournament DX. The demo download link is available in the game's detail page. Simply click the “download demo” button to download the demo to your device. The full retail game will be available to purchase on September 22nd, 2017.

A downloadable demo of #PokkenTournamentDX is coming to Nintendo eShop on #NintendoSwitch tomorrow at 9pm PT/12am ET! pic.twitter.com/8x53oHKMt6

— Nintendo of America (@NintendoAmerica) August 22, 2017

Sources: http://www.nintendo.com/popup/howto/eshopdemo/JzdyUU7cK_oM5qDnatzgA6ZkNY1ktiq3