To correct a Sales Order NOT linked to Contract Record, you must VOID the invoice, edit the Sales Order and then re-invoice in order for the costs to actually impact the contract. If you only edit an already fulfilled Sales Order (and don't void, re-invoice), then the costs will NOT appropriately reflect as cost of contract.
Find the Sales Order and right click, selecting Fulfillments: Order number to Order number - Primary:
This takes you to the Fulfillment record specific to the Sales Order. Use VOID option. This will void the fulfillment and set Sales Order back as Shipped (and unfulfilled). Then you can go back to the Sales Order and link Equipment/Contract to each Item (see #2 in instructions below)
There are two ways you can associate equipment and contracts to Sales Orders and Sales Invoices.
(1) The first way is to check the box that says (1) Add Equipment and Contract below the Items field. Then (2) select the equipment and contract from the drop down lists before you add the items to the Sales Order/Invoice. The (3) Item field can be used to filter by related items to ensure order taker only sees items that can go in this machine
(2) The second way is to add the line items to the Sales Order/Invoice, then double click on each line item and select the Equipment from the Equipment drop down list and the Contract from the Contract drop down list.