An estimate is a realistic guess of what you think a job will cost. It isn't a fixed price.
Because of that, you might need to update an estimate after you've sent it to a customer. It's normal for details to change from time to time.
The estimate now shows the new details.
If you've already turned the estimate into an invoice, you'll need to delete the invoice first.
This puts the estimate back to a Pending status, so you can update it. Then just follow the steps above.