Please note that deleting custom fields is irreversible and you will lose any data stored in those fields.
1. Go to Settings > Custom Fields
2. Select Delete Custom User Field
3. Select the custom field you would like to delete (hold control while selecting to select multiple fields).
4. Toggle the "Confirm Delete" toggle to on.
5. Save settings.
How To: Delete Custom Fields For Orders
Delete Order Item Field:
Similar to product custom fields, to delete a custom order item field, the user can expand the Delete Order Item Field section and select one or more fields to delete. Afterwards, the "Confirm Delete" toggle is turned on and will remove the fields from the Order Item Fields after "Save Settings" is clicked. See below screenshot for reference: