Here's how to change the PST number that shows on your invoices:
Go to the Taxes tab on the left and select Sales Tax at the top.
Choose the tax agency you want to change the number for from the drop down in the top left corner next to Sales Tax. In this case, you'll want to select the PST agency.
Click the Edit [tax agency] drop down on the top right and select Edit Settings.
Enter the new PST number in the Business number (BN) box.
