A Task can be edited by clicking on the task through the Home Screen, Account Detailed Information, Opportunities Detailed Information area or Job Detailed Information area, the same areas where you would add a new task.
Editing a Task from the Home Screen
Editing a Task from Account Detail > Tasks Tab
Editing a Task from Opportunity Detail > Tasks Tab
Editing a Task from Job > Tasks Tab
After opening the Task, you can make any changes you would like. Refer to the Home Screen > Add a Task area for more detail on entering the task information.