Using 2 I2C RGB Color Sensors in Arduino

I want to change one to 0x40

As Passerby said, you can't change the I²C address on the (now obsolete) TCS3414 itself. The manufacturer produced different ICs with different I²C addresses. The list of I²C addresses on that range of colour sensors, is shown in the datasheet:

Therefore if you need to access two of those ICs, you need to consider other options, including:

• Use multiple I²C buses, with one TCS3414 on each bus. Since you mention Arduino, you could put one sensor on the normal hardware I²C bus, and then use a Software I2C master library, to add another I²C bus for the second sensor.
• Use an I²C bus translator (see this document from Linear Technology for examples). These ICs can re-write the I²C address to a second identical device "on-the-fly". My answer on this very similar question also applies here.
• Use either a "home brew" or ready-made I²C bus isolator or multiplexer, to switch a single I²C bus master to each of the colour sensors, one at a time. That would require you to switch the I²C bus to connect to sensor 1, before using that one. Then disconnect from sensor 1 and connect the master to sensor 2 (you may need to add a small delay here, depending on bus capacitance). Repeat as needed.