UPDATE tblBulkPurchases
(tblBulkPurchases.PO_Number,
tblBulkPurchases.Quantity,
tblBulkPurchases.Transaction_Number,
tblBulkPurchases.Quantity_Remaining,
tblBulkPurchases.Unit_Price,
tblBulkPurchases.Software_Description,
tblBulkPurchases.PO_Date,
tblBulkPurchases.PurchaseCostCenter,
tblBulkPurchases.HeatTicketNumber,
tblBulkPurchases.PurchaseAccount,
tblBulkPurchases.Transaction_Date,
tblBulkPurchases.Transaction_Type,
tblBulkPurchases.SoftwareShortName)
SELECT
i.PO_Number,
i.Quantity,
i.Transaction_Number,
i.Quantity,
...
FROM INSERTED i
 inner join tblBulkPurchases
  on tblBulkPurchases.PO = i.po 
WHERE Transaction_Type = 'Bulk Purchase'

CREATE TRIGGER [dbo].[trUpdateBulkPurchases]
ON [dbo].[tblTransactions]
AFTER UPDATE
AS
BEGIN
UPDATE tblBulkPurchases
  set PO_Number = i.PO_Number
     , Quantity = i.quantity
...
FROM INSERTED i
inner join tblBulkPurchases
on tblBulkPurchases.Transaction_Number = i.Transaction_Number
WHERE Transaction_Type = 'Bulk Purchase' 
GO