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.
Tom Howarth VCP / VCAP / vExpert
VMware Communities User Moderator
Blog: http://www.planetvm.net
Contributing author on VMware vSphere and Virtual Infrastructure Security: Securing ESX and the Virtual Environment
Contributing author on VCP VMware Certified Professional on VSphere 4 Study Guide: Exam VCP-410
