To edit sales orders go to orders page and click on the pencil icon on the left side of the screen.

At each stage of the sales orders, editable parts are different.

Draft

At this stage, you can edit all parts of your sales orders.

Finalized

You will no longer be able to edit the customer, currency totals on the sales order.
Pending

At this stage, you can do the same changes as in Finalized stage.

Picked

At this stage, your order is fulfilled and you can edit the ship from location, issue date, shipment date, assignee but you can’t remove line items.