The assignee can be updated for a set of tasks simultaneously using group actions. In any view of the tasks module, click the Group Actions button near the top of the module and select Update Assignee to open the pop-up wizard.
The pop-up will list the available tasks. Select the tasks by clicking on each, or click the Select All button to select all the listed tasks. To remove a task from the selected column, click on the corresponding . When ready, click the Next button. In the next screen, select the intended assignee from the list of available users. Click Submit to continue with the assignee change.
Note that this group action feature is available to users that have access to the tasks module. The list of tasks provided under a group action will be those for which the user has access. Depending on the settings of the users project permissions, they may be able to see all project members or a subset.