Why does updating Client info like an email address not update the email address for previous invoices?

Thanks for reaching out to the Community space, @tania6.

When updating a clients email address, this will not make a change on the previously created invoices. It will only reflect on the newly-created transaction. That's why it doesn't update.

If you'd want to have the new email post on the previous invoices, then you may want to edit the transaction manually.  

In case you'd want to personalise how your sales forms look, consider checking this article: Customise invoices, quotes, and sales receipts in QuickBooks Online.

Please know that I'm only a post away if you have any other questions or concerns. I'll be here to help. Have a nice day.