The Job Contact Details can be changed using the Edit button on the Jobs Details page. The Contact Details by default are associated with the chosen site. 

However as seen below the default can be changed in the <-Settings->  <-Job Settings-> area to associate the details to the Assigned User.

In addition, in the Email Templates area you can add both Site Contact Details and Job Contact details to the Instruct Email to be sent to the Contractor. (See Below)