Editing an order in Magento actually means creating a new order copied from the one you want to edit. Original order will be canceled when you save the new 'edited' one.
Editing form is identical to new order form except that it is populated with data from original order. You can add items, modify quantities and prices, customer addresses etc. If you change any data for products or customer addresses, shipping method will be unset and you will have to choose it again. Reason behind having to choose shipping method again after every change made to product or customer data is simple. New data may require shipping price and availability to be re-evaluated.
But there are 3rd party extensions available which allow to edit an order without canceling them.
For reference:
https://www.mageworx.com/magento-order-editor.html
https://www.iwdagency.com/extensions/order-manager-m1.html