Mews only allows the changing of the “Company Name” or "Travel Agent" on an Invoice if you are within the “editable history window” which is set by your Administrator (Manager at your hotel with Admin Rights).
If the invoice was closed several days (or more) ago, and falls outside the window, during which you are allowed to make changes to bills/invoices, you will need to contact your manager.
The manager will have to open the “editable history window” to allow you to change the company name that far after closing it. (This is a protection for the accounting department to ensure no one changes invoices without approval)
Once the window is open, you can go open the bill, and assign it to the company you would like to assign it to.
Note: don't forget once you have made the change to the invoice, to tell your manager to close the "editable history window" again.