Go to Settings > System and tap "Date & time". The simplest option is to tap on "Automatic date & time" to select the "Use network-provided time" option, which allows the phone to automatically sync the date and time with the wireless network you're connected to.
If you wish to manually enter the date and time, tap on "Automatic date & time" and tap on "None" to turn this off. Now you can tap "Set date" and "Set time" to manually configure the date and time.
Note: you can also tap "Automatic time zone" for the phone to automatically sync the local date and time from the wireless network you're connected to.