Follow these steps to re-assign a task, i.e. to allocate a task from one resource to another:
Expand all groups until you reach the tasks. To do so click on the arrow next to a group, then expand the resource, then expand the project. Or you can expand groups via the context menu. Note that this command only expands the next lower subordinate group, so you have to click on Expand All until you get to the lowest level, the tasks.
Select the task you want to re-assign.
Click on Re-Assign Task in the ribbon.
Select the resource you want to assign the task to from the address book.