0.5V corresponds to 0x16 or 0b0001 0110. This value needs to be written to the SW6 RUN VOLT register (0x79).
Also make sure that SW6_VTTEN = 0 (VTT mode disabled).
The VSW6_RUN[7:0] bits need to be set appropriately. The datasheet describes this. I believe a value of 0x10 will set the output voltage to 0.5V. 6.25mV steps * 16 = 0.1V. The starting point of VSW6_RUN is 0x00 which gives 0.4V, so a 0.1V offset = 0.5V.