In myBooks, the created Delivery can be converted to Sales Order, by selecting Transactions > Delivery > All Deliveries list page opens,

Select the Delivery that you want to convert to Sales Order,

Now you can find the convert to Sales order option above the Filter By Status, Then the selected Delivery will be converted to Sales order. Also, Under the Actions tab, next to the print option you can see the convert to sales order symbol. Just click on the symbol to convert as Delivery.