From the "ok" prompt, type "boot kernel/unix".
Setting the "boot-file" in the EEPROM to "kernel/unix" makes it the default kernel. Alternatively, you can move away the sparcv9/unix file in which case the boot loader will fall back to the 32 bit kernel. Patch installation may restore that file, though.
Beware: when specifying "boot -s", or any boot command with flag arguments, the standard default kernel/sparcv9/unix kernel will be booted; you need to specify "boot kernel/unix -s".