Setup -> Void Transaction -> Transaction Type: Supplier Credit Note -> Search and choose the one you want and void it.
A Credit Note will have the GL entry(table: gl_trans) type=21 and the Accounts Payable (2100 in en_US-new/demo CoA) with person_type_id=3 and the actual person_id being the id of the supplier. This will also bear an entry in the supp_trans, supp_invoice_items and audit_trail tables as well.
On voiding, the amounts get zeroed out in the supp_invoice_items, supp_trans, gl_trans tables for the specific record(s).
The anatomy of the Void of a Supplier Credit Note is discerned from the diff between the pre and post void backups:
--- fa24_1_pre-void_backup.sql Mon Oct 23 09:18:59 2017
+++ fa24_1_post-void_backup.sql Mon Oct 23 09:19:56 2017
-insert into `1_audit_trail`(`id`,`type`,`trans_no`,`user`,`stamp`,`description`,`fiscal_year`,`gl_date`,`gl_seq`) values
- (32,21,1,1,'2017-10-23 09:12:19','',2,'2017-10-23',0);
+insert into `1_audit_trail`(`id`,`type`,`trans_no`,`user`,`stamp`,`description`,`fiscal_year`,`gl_date`,`gl_seq`) values
+ (32,21,1,1,'2017-10-23 09:19:10','',2,'2017-10-23',NULL)
+ ,(33,21,1,1,'2017-10-23 09:19:10','Voided.\n',2,'2017-10-23',0);
-insert into `1_gl_trans`(`counter`,`type`,`type_no`,`tran_date`,`account`,`memo_`,`amount`,`dimension_id`,`dimension2_id`,`person_type_id`,`person_id`) values
- (41,21,1,'2017-10-23','2100','',25,0,0,3,'1')
- ,(42,21,1,'2017-10-23','5010','Test Supplier CN',-25,0,0,NULL,NULL);
+insert into `1_gl_trans`(`counter`,`type`,`type_no`,`tran_date`,`account`,`memo_`,`amount`,`dimension_id`,`dimension2_id`,`person_type_id`,`person_id`) values
+ (41,21,1,'2017-10-23','2100','',0,0,0,3,'1')
+ ,(42,21,1,'2017-10-23','5010','Test Supplier CN',0,0,0,NULL,NULL);
-insert into `1_supp_invoice_items`(`id`,`supp_trans_no`,`supp_trans_type`,`gl_code`,`grn_item_id`,`po_detail_item_id`,`stock_id`,`description`,`quantity`,`unit_price`,`unit_tax`,`memo_`,`dimension_id`,`dimension2_id`) values
- (2,1,21,'5010',-1,0,'','',0,-25,0,'Test Supplier CN',0,0);
+insert into `1_supp_invoice_items`(`id`,`supp_trans_no`,`supp_trans_type`,`gl_code`,`grn_item_id`,`po_detail_item_id`,`stock_id`,`description`,`quantity`,`unit_price`,`unit_tax`,`memo_`,`dimension_id`,`dimension2_id`) values
+ ,(2,1,21,'5010',-1,0,'','',0,0,0,'Test Supplier CN',0,0);
-insert into `1_supp_trans`(`trans_no`,`type`,`supplier_id`,`reference`,`supp_reference`,`tran_date`,`due_date`,`ov_amount`,`ov_discount`,`ov_gst`,`rate`,`alloc`,`tax_included`) values
- (1,21,1,'001/2017','ABC_112','2017-10-23','2017-10-23',-25,0,0,1,0,0);
+insert into `1_supp_trans`(`trans_no`,`type`,`supplier_id`,`reference`,`supp_reference`,`tran_date`,`due_date`,`ov_amount`,`ov_discount`,`ov_gst`,`rate`,`alloc`,`tax_included`) values
+ ,(1,21,1,'001/2017','ABC_112','2017-10-23','2017-10-23',0,0,0,1,0,0);
+insert into `1_voided`(`type`,`id`,`date_`,`memo_`) values
+ (21,1,'2017-10-23','');