Can anyone tell me if there's a way to update the tax code/tax rate for multiple lines on a sales order? We just received a tax exempt certificate, and the sales order contains over 130 lines. Not sure if this can be done via CSV?

This is possible through CSV import, yeah. This might help you https://3703754-sb1.app.netsuite.com/app/help/helpcenter.nl?fid=section_N451889.html