If you have submitted a P46(car) record to report a new car allocation or replacement car by mistake, there is no automated way of correcting this. The record will need to be deleted and the correct information stored in a new car allocation record for the software to pick up the information and transmit to HMRC via the usual HMRC Filing> P46(car) FBI method.
Correction to existing record
1. Select the incorrect record and click click Edit or Delete to modify the car assigned to the individual.
2. When done modifying or adding the benefit and save the record.
4. Resubmit the record using the HMRC Filing > P46 (car) - PREPARE Submission
Removal of inadvertent allocation having been reported.
In the case of a record being submitted the the HMRC Gateway by mistake (i.e. where a car allocation was made in error) - there is no automated way of submission to correct an incorrect/inadvertent allocation, you will need to contact HMRC personally to advise them so they do not allocate a car benefit to the employee and code the employee's tax record incorrectly.
You will also need to "delete" the incorrect record from the P11D Organiser system which will stop it from being picked up as a reportable P11D return at the end of the year.