This is pretty straight forward if you have small infra with vsphere 5.5, like you have all vCenter components on single machine. It also depends how you want to distribute your 6.5 architecture as 6.x version has PSC controller which isn't there in 5.x. I am sure you are aware of it. The steps would be :
1. Check prerequisites and requirements : https://docs.vmware.com/en/VMware-vSphere/6.5/vsphere-esxi-vcenter-server-65-installation-setup-guid...
2. Backup current configuration.
3. Upgrade vCenter (if you have SSO separetly then upgrade it first to PSC, then upgrade vSphere web client if it is installed separetly)
4. Upgrade Vsphere Update Manager.
5. Upgrade ESXI hosts.
6. Upgrade virtual machine's tools and hardware.
In step 3, if your existing vCenter has all roles on single machine then upgrade path is straight forward. If not then upgrade :
1. single sign on.
2. vCenter
3. vSphere webclient (if installed separately)
For detailed steps : Overview of the vSphere 6.5 Upgrade Process
