There are two ways to reassign a lead to another user, both one at a time or multiple at once.
From the lead's page
To reassign the lead from their page, use the drop down menu in the top right.
From the lead dashboard page
You can use the bulk actions to select and reassign multiple leads to another user. Select the leads using the checkboxes to the left of the leads. Then click "Update Leads" in the top right.
From the window that appears, choose the new assignee from the drop down menu.