To amend the registration number on a stock vehicle, you'll firstly need to load up the stock record via vehicle admin toolkit and check it's at an "amendable" status, the Reg No field shown in the details tab below won't always allow you to overtype/amend.
The vehicle must not have an invoice raised against it, with a sales order, or be delivered, these will need cancelling/un-delivering before you can overtype.
On the "Details" tab there is a "Remove Reg" button, this is used for cherish plate transfers or input errors. Once the "Remove Reg" option is pressed the reg will blank out.
You can then enter the correct registration number and press tab to save.
Please ensure this is entered in uppercase and without spaces