You have some options:
1º downgrade your license to version 5, but not 100% sure if you can since you have a Essentials Kit and I don't know if you can downgrade only the vSphere licenses and let the vCenter license at version 6... anyway I will not recommend you downgrade your licenses;
2º upgrade your hosts to version 6 using the interactive installer and once on version 6, add them to vCenter Server. Take a look here for how to upgrade: VMware KB: Methods for upgrading to VMware ESXi 6.0