How can you change Date and Time on SAMSUNG Galaxy A20?

From your phone, swipe down from the top of the screen, then tap the Settings gear icon.
Scroll down to the System category. It should be near the bottom of the Settings page.
Tap Date & time. It's near the top of the System page.
Tap the Automatic date and time option. Doing so will disable automatic date and time setting, allowing you to adjust both manually.
Tap Set date. This opens the calendar and allows you to select a date.
You can also tap Set time. You can adjust the clock to read the time you want to see (including whether it's currently AM or PM if you aren't using 24-hour time), then tap Done or Save.
Select a time zone if necessary. If you want to select a time zone that's different than your current time zone—or if your current time zone is incorrect—tap Select time zone and then select the time zone.
Enable 24-hour time. If you want your time to display in 24-hour format, you can enable this setting by tapping the grey switch.