When creating a document with QuoteMachine (Quote, Sales Order, Invoice, etc.), you will always have an employee assigned to it. The assigned employee to a newly created document will default to the currently logged-in user.
You can change it when the document is in draft or edit mode by clicking on the modifier icon.
How to change the assigned employee to a finalized document
Once a document has been finalized (Mark as Done or Sent to the customer), it is still possible to change the assigned employee.
You can do so by clicking on the "edit" option on a document, and then proceed to the change.
Important note if you're using the Lightspeed Retail integration
Changing the assigned employee from QuoteMachine to a document that has already been exported to Lightspeed Retail does not change it in Lightspeed Retail.
You can refer to the following help article from Lightspeed to learn how to replicate this from Lightspeed Retail. Editing a sale (links to Lightspeed's support center)