My condolences to you. You will need to need to apply for a Grant of Probate (if there is a Will) or Grant of Letters of Administration (in Intestacy) that will authorize you to deal with the deceased's assets. Take note that Grant of Probate is usually given to direct family members, and the process is more complicated if you are not directly related to the deceased.

LTA recognises the authorisation letter and allows the representative to either de-register or transfer the ownership of the vehicle.