Modifying a chip on a bought voltage measurer
It's easy to write to the 24C02, which is just an I2C SEEPROM memory. Anything that will act as an I2C master will work. 

Working out what to change in the 256 bytes of memory will be a more difficult problem I suspect, and very specific to this particular end product. It may be stored in a very naive way, or (like some stuff I've developed) there may be redundancy and self-repair algorithms involved, along with the checksums or CRCs to detect problems.