Sharepoint - Sign in as different user in SharePoint 2013

•Locate the file \15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx and open in a text editor.

•Add the following element before the existing element with the id of “ID_RequestAccess”:

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

•Save the file and close. •Open your SharePoint site.

You will get an option for Sign In as Different User below "About Me" option

There are several solutions to your problem as Microsoft has removed the Sign in as different user link.

1. Start your browser as a different user
2. Use the _layouts/closeConnection.aspx?loginasanotheruser=true page
3. Edit the Welcome.aspx to include the link again (Sign in as Different User and SharePoint 2013)
4. Use JavaScript as a Bookmarklet (SharePoint 2013 – Sign in as Another User)

Official MSDN article about this problem: "Sign in as Different User" menu option is missing in SharePoint Server 2013. And remember: It's a feature, not a bug ;-)