(the info below is without warranty)

• get the latest firmware from ftp.digital.com (choose your machine on the left)
• download the firmware file (in my case it was `as255_v8_1.exe')
• boot linux
• format a floppy (`fdformat /dev/fd0H1440')
  *don't* put a filesystem onto it - you don't need it!
• put the firmware file onto the disk - starting with an offset of 512 byte:
  `dd if=as255_v8_1.exe of=/dev/fd0 bs=512 seek=1'
  the next step is somehow a hack. maybe there is a better way to do it - pls inform me! you need to boot from the disk (directly in the srm console) - but currently there is no boot stuff on it.
• create an empty (hex) bootblock:
  `dd if=/dev/zero bs=512 count=1 | xxd boot_block_empty'
• determine the size of the firmware file
  `ls -la as255_v8_1.exe'
  divide that size by 512 (decimal) and calculate the hex value of it.
• edit the file `boot_block_empty' you created in step 6. you have to jump to offset 1e0 (the line before the last line). the next is a bit complicated. you have to reverse the byte order of the hex value calculated in step 7 in a long int (8 byte - 64 bit) order. example:
  your value from step 7 was: a09 (= 0000 0000 0000 0a09)
  the value you have to enter is: 090a 0000 0000 0000
  hope everyone understood...
• edit the next long int
  enter: 0100 0000 0000 0000
  this is the offset the file starts.
• make sure that the next long int (at offset 1f0) is:
  0000 0000 0000 0000
  this is a flag - don't know what it is used for.
• edit the last long int. this is a checksum. it is the addition of all long ints (8 bytes) before. pay attention on the REVERSE ORDER!!!
  in my example it is: 0a0a 0000 0000 0000
• exit editor and write the bootblock back to disk:
  `xxd -r boot_block_empty | dd of=/dev/fd0H1440'
• halt linux -> go to the srm console and boot from the created disk: `boot dva0'
• the machine/srm seems to reboot - the the update menu should appear.