Ensure that the backpack pins are straight and fit in the LCD module, then solder in the first pin while keeping the backpack in the same plane with the LCD. Once you have finished the soldering work ...

In addition to the ICSP, you get the pins in UART order and an I2C header. Vcc is switch selectable for 3.3 or 5volts. The reset switch has been moved to the edge plus two additional ADC pins.

i2c pins are open collector, so their only connection to +V is the pull-up resistor. For _most_ parts, input high is considered around 0.6*Vcc, so for a 5V part that’s around 3.0V.