Trying to get FNB to re-address

My wife has had an FNB account for many years. After 43 years we sold our house and moved to an apartment. For more than 6 years she has been trying to get FNB to change our home address but it seems to be something that they simply are unable to get right. It has caused an untold number of delays in financial transfers and other deals or situations when FNB are requested to supply information. How do we resolve this?