Thank you for reaching out to the Community. I can help you modify your tax agency in QuickBooks Online. Here's how:
Go to Taxes.
Under the agency name, choose Edit.
Enter the new name, then select Save.
Editing the Sales Tax Agency's name is possible through the Taxes page. However, the agency name that'll show on your sales forms would still be the old name or the initial name that you've entered.
For additional information, you can check out these articles: