PA7 to PA0 are all available as GPIO. Look at the code and the hardware design to be sure they are not being used for something else, pick one that is free.
Use OEA = 0xXX; where hex value XX sets the pins to 0 for inputs and 1 for outputs as required for the 8 ports
Then use a bit mask to toggle the port you want.
IOA |= 0x01; // Sets port PA0 high without changing other PAx ports
IOA &= 0x04; // Sets port PA3 low without chaging other PAx ports