The screen is one of the most significant drains on the battery. The good news is, the power the screen uses can be reduced by turning down brightness.
Open Control Center by sliding up from the bottom of the screen. Here, you can access the brightness control and reduce backlight strength.
Another option is to reduce the auto-lock time, which will turn off the screen sooner after you stop using the device. Open Settings, General, then Auto-Lock and select a shorter time period to help save power.
The iPhone changes brightness automatically based on current ambient light, which can save you time and effort as well as battery. This setting is activated by default, but if you’d prefer strictly manual control of the display you can disable it from Settings > General > Accessibility > Display Accommodations. Just toggle Auto-Brightness to the off position.