Complex sentence: Erasure of old programs was carried out by either over-writing them with a new download, or bulk erasing them electrically (for EEPROM) which was slower, but could be carried out in-situ.
Split into simpler sentences: Erasure of old programs was carried out by either over-writing them with a new download, or bulk erasing them electrically (for EEPROM). The latter method was slower, but could be carried out in-situ.