If you need to export Overview of Site Audit report showing the list of issues for your site, you can use this option:
Click "Print" button and choose location on your computer where Overview in PDF will be stored and click "Print". Now PDF Overview of Site Audit report is saved on your PC.
If you need to export the list of issues, not just their amount, this can be done for each issue separately.
For this click on the issue you are interested in, open the list of pages if was found on and click Export choosing particular data for exporting:
Please, mind there's currently no way of exporting detailed list of all issues with specific pages in a single go.