Question on pin management using PCA9534PW IO Extender
You set the pin mode by setting the configuration register. 

Wire.beginTransmission(0x20);
Wire.write(0x03);                   // Configuration Register
Wire.write(0xfe);                   // IO7-IO1 Input, IO0 Output
Wire.endTransmission();


Then write the output port register with HIGH and LOW.

Wire.beginTransmission(0x20);
Wire.write(0x01);                   // Output Port Register
Wire.write(0x01);                   // IO0 HIGH
Wire.endTransmission();
delay(500);
Wire.beginTransmission(0x20);
Wire.write(0x01);                   // Output Port Register
Wire.write(0x00);                   // IO0 LOW
Wire.endTransmission();


But using a library is much easier. See, for instance, https://github.com/alotaiba/PCA9534.