The codes in a Trigger will be executed by database engine automatically and can't be execute the trigger manually.
If you want to execute the codes manually, then, you have to use
stored procedure.
Instead of doing this:
Create Trigger Invoice_Trigger
on InvoiceTable After InsertUpdate
Update Invoice set invoiceId=20
You might have to do this:
create proc invoice_proc1
as
Update Invoice set invoiceId=20
