Start with a visit to the vet, explain the problem and have a full blood panel with thyroid before anything.
Dagodingo has the right of it.
Go to the Vet - first port of call. Make absolutely sure there is nothing wrong which is causing your Basenji to turn crabby. Once you have eliminated any/all possibilities of 'discomfort' - loss of hearing, sight, something affecting general well being - then ask yourself if there have been any changes to lifestyle. Someone new in the household, someone lacking in the household.
It can be quite a small thing, but it could upset the equilibrium especially in an elderly Basenji.
But then reply to some of the questions -
How old is the dog? Male or female? Were there any signs of aggression prior to this? How long have you had the dog?
so this forum can start to suggest things other than trainers - at the moment these seem to me to be inappropriate