Sensor TSL2591 with Arduino Mega
Looking at the datasheet, the TSL2591 does not appear to have a comfigurable address mechanism, therefore you are most likely obliged to use an I2C Multiplexer.

One example of such a device is the TCA9545A, Low Voltage 4-channel I2C and SMbus Switch With Interrupt Logic and Reset Functions, that can split one I2C line into 4 buses.



They are very easy to use and the pinout (and datasheet) are self explanatory.

Don't forget that you will need 3k3&nbsp;&Omega; resistors on the INT, RESET, SDA and SCL lines from the Arduino and on each of the INTx, SDx, and SCx lines from the TCA9545A: