You can change the time manually on an iPhone through the Time and Date section in the phone's settings menu. However, an iPhone's time syncs automatically with its network provider's servers, and thus a consistently incorrect time or date may indicate a communication problem.
Before manually changing the time, ascertain that the phone is communicating properly with the cellular network provider's servers. To do this, unlock the phone, and launch the Settings app from the springboard. Open the General settings, located near the top of the settings menu. Then, choose the Date and Time option. Within this menu, the Set Automatically toggle switch should be turned on. This not only allows the phone to synchronize its time with the network provider's universal clock, but also allows the phone to change the time automatically when the user crosses into a new time zone. It also allows automatically changes the phone's clock during daylight savings time. If any of these features do not work, Apple recommends contacting the customer support department of your cellular provider.
To change the time manually, turn off the Set Automatically toggle switch. This reveals options to change the phone's time, date and time zone. Set the phone to the desired time, and then close the settings app to complete the change.