Hello.
Before any upgrade you should perform a full backup of all VMs and ESXi Host configuration.
The usual is to upgrade the vcenter server (if any) and then the ESXi host, for security reasons you must validate the compatibility of your versions (vcenter server and ESXi Host).
https://interopmatrix.vmware.com/Interoperability
It is recommended to use the server manufacturer's custom image. In your case you should use the Lenovo custom image (Lenovo acquired the intel server part from IBM).
It is also advisable to update the Firmware of the server.
https://vmware.lenovo.com/content/custom_iso/6.7/