Are you referring to unticking invoices on the Invoices page? If so, you can select all invoices by ticking the box before the Date column and then, untick the box once more to deselect the invoices. Here's how:
Click the Sales tab.
Click on Invoices.
Tick the box before the Date column, and then tick the box once more to deselect the invoices.
If you're referring to deleting invoices, you can select multiple invoices under the Invoices tab and then, click Batch Actions to delete them.