To re-assign a lead to another user follow these steps.
Find the lead you wish to re-assign to another user
Navigate to the details tab on their profile
Select the 'Assigned to' box, and then select the user you wish to re-assign the lead to.
I'm not able to re-assign a lead? Make sure you have permission to re-assign leads. You can check this with your manager or by going to settings > sales teams > selecting your location > selecting the user you wish to check > scrolling down and ensuring the "Reassign Leads" Permission is turned on