I need to convert multiple Sales Invoices to Delivery notes how to do that?
I think ERPNext expects you to do the Delivery Note first and then the Sales Invoice. Consequently, there is no direct way of making a single delivery note from multiple Sales Invoices. Your easiest bet is to use an underlying sales order and then make Delivery Note and Sales Invoice from that.
Or you can try and see if you can add Sales Invoice as one of the options when you click on the Get Items From button in the Delivery Note document.
Hope this helps.