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 Program table contains the following custom fields:
Text
Custom 01, Custom 02, Custom 03
Lookup
Custom 04, Custom 05, Custom 06
Multivalue lookup
Custom 09
Date
Custom 08, Custom 21, Custom 22
Numeric
Custom 07, Custom 19, Custom 20
Checkbox
Custom 10, Custom 11, Custom 12, Custom 13, Custom 14, Custom 15, Custom 16, Custom 17, Custom 18, Custom 23, Custom 24, Custom 25