Go to Orders & Quotes > Orders and you will see a table of your Orders.
This table will only show you the first 200 orders. You can narrow down the selection by typing in the search bar:

OR
using the drop down to filter results:

Either way, once you see the relevant order in the list you can click the magnifying glass to view said order.

You will be taken to the screen where you can view and edit the order.
Tabs
Make sure to note that there are different | Tabs | on the order screen.
Each containing a view that matches the tabs name.
References
These are the identifiers of an order such as the Order ID and customer email address associated with said order.
Click the [Edit] button to edit the orders references. Some will be locked and un-editable, this is normal.
Internal Notes
These are notes you make on an order. They are only visible internally. As such these are not sent to the customer or suppliers.
You can easily add, edit and remove order notes.
Order Details
These are the lines that make up an order.
You can easily add, edit and remove order lines.
Order Summary
This will give you the overarching financials of an order. Like the Sub-Total, VAT / Tax (if applicable), Delivery etc.
Delivery & Billing Details
Edit the order delivery and billing details in these sections with the appropriate [Edit] buttons.