You go to Providers>Parent Transporters (Screenshot below.)
It will bring up this screen (Screenshot below)
On this screen, highlight the parent whose vendor # that you need to change. In this case I've highlighted 11-301.
Then you will click on this edit button (Screenshot below. Highlighted in red.)
Selecting this edit button will cause the parent information that was grayed out to become black and active, like so (screenshot below)
You can then edit this field. Click the apply button when you are finished, which has also become black and active instead of grayed out (Screenshot below.)
The parent Vendor Number has been updated.