Hi,
Before any new beginning you have to mind the following informations :
1) check if your windows version is compatible with the vCenter server version you want to update to => VMware Knowledge Base
2) Check if one of your ESXi has enough resources to host temporary 2 vCenter (the old one will be shutdown by the end of migration)
3) Reserve a temporary name and IP address (only if you want to update to VCSA (Linux)
Then,
1st step :
You have to mount the ISO on your actual/source vCenter server,
then go to the folder Migration Assistant on D:\ then run the "VMware-Migration-Assistant.exe". It will opens a CMD window, let the precheck run until it mentions that's ok you can migrate.
2nd step :
Once done, go to your PC or a third part device.
Mount the ISO on your computer then go to D:\ to the folder "vcsa-ui-installer\win32\" then lauch "installer.exe"
Let me know if it works