If you're lucky, eitehr th microcontroller is old
enough not to
 have any
 security features, or they're not used. In which case you remove
 the chip
 from the PCB and stick it in a suitable programmer. 
    Even the original Intel 8051/8751, circa 1980, has security
 bits. :-(  There are ways to get around (at least) those, though. 
 
 Yes, but IIRC the 8048 series (common in classic computer stuff)
 doesn't...