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 Person table contains the following custom fields:
Text
Custom 01, Custom 02
Lookup
Custom 03, Custmo 04
Memo
Custom 05
Date
Custom 06
Numeric
Custom 11
Checkbox
Custom 07, Custom 08, Custom 09, Custom 10