If we've not made the correct vendor assignment, or perhaps you want to change it to match a new name, then you want to change a vendor on a transaction.
Note: we currently don't support editing vendors.
Here's how to edit the vendor on your transactions:
Go to Transactions
Navigate to the transaction side-bar on the transaction in question (click on the blank space)
Make sure the transaction is not "Finalized" (that's when there's a green check on it).
If it's "Finalized" click the green check and it will revert to "User assigned" and be editable.