from memory, load up the management interface, fog settings (top of screen), pxe boot menu (left of screen)
i've found this section a little flakey on occasion occasional getting a TFTP error when setting the passwords, but normally a long delay and a success message, or just instant success. Part of the problem for me may be that i'm running behind a proxy so parts of the management interface don't work properly causing delays when i click on menu items.
The general advice when having password issues seems to be to set everything to the same password intially. There actually of lot of places where passwords need to be reset if you do this:
fog user account on the operating system
fog user for management interface
storage node password
pxe boot menu
there are also the .php configuration files, one for general config, and i think a seperate one for multicast settings.
...i think anyway, it's been a while since i moved it to a new server and had to find them all.