I'm sorry but have you bothered to actually read any of the links that people have placed in their replies for you?  Or do you want us to put a point by point list in the thread for you?

There is an excellent VMware Paper on upgrading from vSphere 5.5 U3 to 6.5U1. 

it goes though the process. 

Yes you can still use a Windows based vCenter for vCenter 6.5, but why would you when there is an excellent migration tool that will move you to the fully functional Appliance whilst keeping all of your settings.