To what extent can you not boot? Will the machine at least kick in the ROM update? If that's the case, hold down both mouse buttons to get to the Early Startup menu, as usual. The power light will flash very rapidly to indicate you're there. Hold down Ctrl + Alt + Del + Backspace and the bootmenu should switch to a 31KHz screen. Confirm DoublePAL/NTSC in the display menu, boot without a startup-sequence, and then you can rename your files from the mini-Shell.
Alternatively, add NOROMUPDATE to your Setpatch line in the startup-sequence.
