Firstly you need to download the vCenter 6.5 installation ISO at VMware website, then put it into vCenter host. Then execute autorun.exe and click Install button.
Input vCenter Sign-On password, then Next.
Select what type configuration data needs to migrate, then Next.
Verify the target location for vCenter 6.5 upgrade, then Next.
Select verify setting and then clicks Upgrade.
It starts to upgrade progress.
Finally the upgrade successfully completed, now you can launch vSphere Web Client.