To write on the I²C bus, the controller sends a START condition on the bus with the address of the target, as well as the last bit (the R/ W bit) set to 0, which signifies a write. After the target sends the acknowledge bit, the controller then sends the register address of the register to which it wishes to write. The target will acknowledge again, letting the controller know it is ready. After this, the controller starts sending the register data to the target until the controller has sent all the data necessary (which is sometimes only a single byte), and the controller terminates the transmission with a STOP condition.

#SCPS2342395 and #SCPS2345022 show an example of writing a single byte to a target register.