Check the volume name of your USB stick:
diskutil list # check for something like /dev/disk2
Then erase what you have on the stick currently:
diskutil eraseDisk FAT32 LINUX /dev/<DISK>
Unmount the disk:
diskutil unmountDisk /dev/<DISK>
Write new ISO on the disk:
sudo dd if=<path to downloaded .iso> of=/dev/<DISK> bs=1m
That's it! :)