After you upgrade to ESXi 6.5, you must apply a vSphere 6.5 license.
When you upgrade ESXi 5.5 or ESXi 6.0 hosts to ESXi 6.5 hosts, the hosts are in a 60-day evaluation mode period until you apply the correct vSphere 6.0 licenses. See About ESXi Evaluation and Licensed Modes.
You can upgrade your existing vSphere 5.5 or 6.0 licenses or acquire vSphere 6.5 licenses from My VMware. After you have vSphere 6.5 licenses, you must assign them to all upgraded ESXi 6.5 hosts by using the license management functionality in the vSphere Web Client. See the vCenter Server and Host Management documentation for details. If you use the scripted method to upgrade to ESXi 6.5, you can provide the license key in the kickstart (ks) file.