I am using 6.7 VM center and want to upgrade, because OEM not support for existing version.
Please support here to upgrade current version.
You don't mention if it is a Windows based vCenter or VCSA. Here is a link so you can have an idea of options and additional information.
I am not sure if you did the upgrade. Per my experience with vCenter in Windows, the best upgrade I did to vCenter was to deployed it as a VCSA. In this way I don't get patches push down on the system and rebooted at the worst time possible.
You required migrate Windows based VC to Appliances based VCSA
I would just deploy a new V7 VCSA from scratch. Copy the configuration across, get it how you want it, then move the hosts across one by one. If you have spare capacity, you can stand up a new V7 cluster, do your testing, then when ready, simply use Advanced Cross vCenter vMotion (which is built into the UI in v7.0) to migrate the VMs non disruptively from the 6.7 cluster to the 7.0 cluster. No Enhanced Linked Mode required, the migration works across different SSO domains. That's what we did and it worked like a charm.
You'll get a nice clean install of VCSA, and learn a lot in the process! 😊
Windows vCenter is no longer supported from version 7 so you will have to deploy the VCSA.
The easiest way I have found is deploy a new VCSA and get it configured and working and then gradually disconnect hosts from the old vCenter and add then to the new vCenter. This allows options to "back-out" as in disconnect form new and re-connect to old for a short period of time.
You will find the new version different but it's no just your OEM but VMware that no longer support version 6.x, also bear in mind that 7.x will go end of general support 2025-04-02 so if you can go to version 8.x then this would be much better for you. Remember to check hosts and against the HCL https://www.vmware.com/resources/compatibility/search.php for both 7.x & 8.x compatibility.
Some useful planning links