Enable Sign in as different user option in SharePoint 2013

By default in SharePoint 2013 there is no option for “Sign in as different user”.

To enable this option you have the update the SharePoint welcome template.

C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx

Add the following code after the <SharePoint:MenuItemTemplate runat="server" id="ID_RequestAccess" finisher tag />.

<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser"
  Text="<%$Resources:wss,personalactions_loginasdifferentuser%>"
  Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>"
  MenuGroupId="100"
  Sequence="100"
  UseShortId="true"
/>

Source

How to enable “Sign in as different user” option in SharePoint 2013