Activate the new Discord user interface for Discord Mobile:

1. Send the Sparkles emoji to any server.
2. Once sent, tap on the Sparkles emoji to activate the description box of the emoji.
3. In that box, tap and hold on the Sparkles emoji image until the new layout option appears.

To turn it off if you don't like it:

1. Tap on your profile on the bottom right.
2. On the top left, tap on the gear to open App Settings.
3. Tap on Appearance.
4. Turn off the toggle for "Show New Layout".