You, can use vSphere 6.0 client to access ESXi 6.5.

There are 2 ways to download the vSphere client.

1) Access the Existing ESXi 6.0 by browsing it from browser. From there you can download the vSphere client.

2) Download it from the link mentioned VMware Knowledge Base  to download vSphere leint C#.

-Sachin