There are two ways to accomplish this.
Method 1: From the View Leads Page
Click on the 'Leads' Tab to load the View Leads Page. Use sorting and filtering to locate the lead(s) to be assigned/re-assigned (review this article for details on filtering and sorting).
First, use the check-boxes located on the far left-hand column to select leads.
Next, click the small arrow icon at the top of the check-boxes column to bring up a menu.
The Assign Leads pop-up window will appear. Select the name of the user to assign leads to that user:
Method 2: Batch Processing