In QuickBooks Online (QBO), the system will only send an email invite during the initial set up. You can update the email address from their profile but you can't re-invite them to complete their W-9 information. As a workaround, you can add them again as a new contractor and make the other one inactive.
To make them inactive, please refer to these steps:
In QBO, go to the Workers menu at the left pane to get to the Contractors page.
Locate the contractor, then click the drop-down under their name, then pick Mark as inactive.