In the previous versions of SharePoint we always had the option to sign in as a different user. since SharePoint 2013 we only have the option to sign out.?

For the majority of users this is no problem at all because they only have 1 account to log in, but for an administrator this can be a issue as they usually have a separate administrator account.

To solve this we have a few options:

1. Start the browser as a different user with runas
2. Create a shortcut with “/_layouts/closeConnection.aspx?loginasanotheruser=true”  at the end
3. Update the Welcome.acsx file in the controltemplates folder with an extra menuitem

The last option should never be done directly. This is an option if you have a custom masterpage where you can reference to your own welcome.ascx. the extra code to add in de welcome.ascx is. <SharePoint:MenuItemTemplate runat=“server” ID=“ID_LoginAsDifferentUser” Text="<%$Resources:wss,personalactions_loginasdifferentuser%>" Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>" MenuGroupId=“100” Sequence=“100” UseShortId=“true”/>

See this Microsoft KB 2752600 article for the official work around