After you apply the April 14, 2015 update (KB2889923), Lync 2013 will be upgraded to Skype for Business.
You can still use the Lync client user interface after you apply this update for Skype for Business. Here is info on how to switch between the Skype for Business and the Lync client user interfaces for Office 365 users and Lync Server 2013 users.
To install the update follow these steps:
- Make sure Lync build is at 15.0.4569.1506 (KB2863908: March, 2014 update).
- Apply update KB2889923 April 14, 2015.
- Apply update KB2889853 April 14, 2015.