Tasks may be completed from either the Tasks Tab, or from within the Lead Detail. In either instance, the process is the same:

To complete a Task, simply click the green checkbox icon next to the Task name. You will also be able to enter a optional comment related to the task at this time. Click the "Complete" button to complete the task, or click "Cancel" to cancel the process.

From the Lead Detail:

From The Tasks Tab:

Completing the Task and adding a Comment from the Tasks Tab: