The installer will remove the older components, import the data, and perform a new installation of vCenter 6.5 and its components. The amount of time the upgrade would take to finish successfully is very dependent on the amount of data that needs to be imported into the new installation and don't be surprised if the upgrade runs for more than 30-40 minutes. Once the installation is complete, you will be able to access vCenter 6.5 using vSphere Web Client. If there is more than one vCenter to upgrade, the procedure remains the same. However, the vSphere 6.5 Web Client would still let you view and manage the older version of vCenter.