You can drag and drop jobs in the day view to different Mechanic columns to assign the job to the Mechanic. You are free to do this as many times as you want.
Note that it is best if you drag n drop the job boxes to positions in the mechanic column that is close to the mechanic name header (where the arrow above is pointing is good). Dropping the job boxes lower down the columns may not work as the field that registers the drag n drop which are invisible to you do not extend as far down.