To rename a custom field, open the data entry window, right-click the data field and choose Rename Field from the popup menu. You can also rename fields from the 'Database Setup' window (Tools=>Database Setup).
The Borrower table contains the following custom fields:
Text
Custom 01, Custom 02
Lookup
Custom 03, Custom 04
Multivalue lookup
Custom 12
Memo
Custom 05
Date
Custom 06
Numeric
Custom 11
Checkbox
Custom 07, Custom 08, Custom 09, Custom 10