From setup page navigate to Objects & Fields -> Object Manager.
On clicking ‘Object Manager’ you will be redirected to the Object Manager screen.
By clicking on the ‘Order’ module, you will be navigated to the Order details page. Click on ‘Page Layouts’ to manage the order page layout.
On clicking ‘Page Layout’, you will be presented with the Page Layouts page. Click on ‘Orders Layout’ to edit the layout.
By clicking on the ‘Order Layout’ link, you will be redirected to the Edit view of the Order Layout.
Now drag the ‘Related Contact’ field to the Order Detail section and click on the ‘Save’ button to save the changes.