You can not change the name of a field in FIMS, however you can use some fields for a different purpose. For example, if you aren't using the County Code field on the Fund1 tab, you can use it for another specific purpose. You would need to add the codes to the County table through File Maintenance > Funds > Fund Code Maintenance.
Be aware that those fields would only be available in reports that pull from the Fund table.
When using the field for a different purpose it is always a good idea to enter use help by clicking F2 and adding explanatory text.