To preserve the integrity of the estimate, you are not allowed to edit an already approved estimate. To proceed, you have two options:

1. Issue an invoice from the estimate. When you issue an invoice from the estimate, you have the option to make changes.
2. Click the 3-dot icon to Reject the approved estimate and then again to make a copy of the estimate. You can make changes to the copy and mark it as approved or resend it to the client for approval.