Don't panic, it's super easy to fix. First we'll add the correct supplier to the invoice then go over how to prevent it from happening.
Fixing the Invoice
This can be done while the invoice is in the Approval Tab. Select the invoice in question, and change the supplier from the Supplier drop down.
If you can't find the supplier you're looking for, you may need to create it in the Suppliers section.
Preventing a supplier being incorrectly added to an invoice
To stop Lightyear from automatically using that supplier next time, you'll need to remove the previously saved 'Best Maps' relating to that incorrect supplier. To do this,
Go to Suppliers > find the supplier > Edit.
Scroll down to the Sender Detail section to see a list of all approved email addresses for this supplier.
Just click the trash icon next to the email address this invoice came from to prevent future invoices from this contact being mapped to this supplier.
To find out the map number & sender's email address, you just need to look at the invoice history for the invoice that had the wrong supplier.
Pull up the invoice in your Approval Tab and go to the Activity section.