How do I change the I2C address on the LCD backpack?
As mentioned in the page you linked, jumper together one of the address jumpers. For example, above A0 are two pads. Solder a wire across them (vertically - between the two pads above A0). I am guessing the address is pulled high by a resistor, so that will probably make the new address 0x3E, but you could run the I2C scanner to check that.