If you go to, Settings-lock screen and security-Always on Display, you can select Digital, Analog, Calender, Picture...etc. You can also change the time it turns on and off. There is an alternative, you can do a theme selection that changes everything, or just the Always On Display. Go to Settings-wallpapers and Themes, and at the bottom it gives you options. Select either Themes, or AOD(always on display) for pay and free options.
I hope this was helpful