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