There are several places where you can reassign work and its' tasks to a colleague. One way to reassign work and the tasks is to do this within your work view. Go into the options menu of that work, and select Assign to colleague.
Once you click on the options menu, it will bring up a module that allows you to reassign to your colleague and also choose if you'd like all the tasks and any notes that are present in that work item to be reassigned as well.
You can also do the same within your To Do View and the Work itself. Much like the Work View where a module was pulled up for you to reassign the work, tasks and any notes, this will also occur when reassign in the To Do view and the Work item.