Go to the Leads tab, locate the lead and click on the lead's name at the top left side of the lead card to open the lead card view. Then click the Edit button located to the top right to open the lead form. In the lead form click the Lead Source field to open a menu, select the lead source you want to assign. This will add the option to the left to select a source contact, click the Source Contact field to view the menu of contact and select the contact. To complete scroll to the bottom of the form and click Save.