When you install the Update Manager server, the Update Manager client interface becomes automatically enabled in the vSphere Client. In earlier releases, after you install the Update Manager server component on Windows, the Update Manager client interface becomes automatically enabled in vSphere Web Client. After the deprecation of the Adobe Flash Player, however, using the Flash-based vSphere Web Client is not recommended. So, starting with vSphere 6.7 Update 3m, when you use Update Manager on Windows, you can and must access the Update Manager client interface through the vSphere Client.
For detailed information about the Adobe Flash Player End of Life (EOL) and its impact on different VMware products, see the VMware knowledge base article at https://kb.vmware.com/s/article/78589.
For information about the Update Manager client interfaces, see Overview of the Update Manager Client Interfaces.