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 Contents table contains the following custom fields:
Text
Custom 01, Custom 02, Custom 03, Custom 18, Custom 19, Custom 22, Custom 23
Lookup
Custom 04, Custom 05, Custom 06, Custom 20, Custom 21, Custom 24
Multivalue lookup
Custom 35
Memo
Custom 34
Date
Custom 07, Custom 25, Custom 26
Numeric
Custom 08, Custom 09, Custom 27
Checkbox
Custom 10, Custom 11, Custom 12, Custom 13, Custom 14, Custom 15, Custom 16, Custom 17, Custom 28, Custom 29, Custom 30, Custom 31, Custom 32, Custom 33