Custom Fields can be managed and deleted from the Custom Fields page.
Once a custom field has been applied to transactions, it cannot be deleted. To prevent further use, the custom field can be deactivated.
Security
Users with the following permission can delete a custom field:
Administration → Custom Fields → Delete Custom Field
Delete a Custom Field
Follow these steps to delete a custom field:
Click steps to expand for additional information and images.
1) Navigate to the Custom Fields page.
A) Open the Admin application.
B) Expand the System category.
C) Select Custom fields. 
The custom fields page will open.
2) Click in the desired Custom Field row to open the record.
3) Click the menu icon (...).
4) Click delete.
5) Review the warning message, then delete.
If the custom field has been applied to transactions, it cannot be deleted.
To prevent further use, the custom field can be deactivated.