If you want to upgrade to Skype for Business Server 2015, and don’t have a purely Lync Server 2013 environment, you will have to follow the migration path. The recommended path is to migrate from OCS or Lync Server 2010 to Lync Server 2013, then complete an in-place upgrade.
- Skype for Business Deployment Step by Step (Greenfield Deployment)
- Skype for Business In-Place Upgrade Step by Step
- Step by Step Deployment and Migration of Lync Server 2013
- Lync 2013 RTM migration from Lync 2010 Step by Step – Part 1 | Part 2
- Migration to Lync 2013 (Technet)