Reassigning a Lead is easily done, but can only be done in the Web Portal.
In the Web Portal go to Sales > View Leads > select the Pipeline of the lead in question > select the Edit pen for the Lead.
In the pop up modal, you can change the settings for the Lead, including the User that it is assigned to. Simply scroll down to the Assigned To field, and select the assignee from the dropdown list. Once that is done, select Save or Save & Exit if you have nothing else to edit for the Lead.
