Find the specific release for "6.5 U3" (often listed by date, such as October 2019 or later).
You will need an HPE Passport account to access the download. download hpe custom image for esxi 6.5 u3
Search for "HPE Custom Image ESXi 6.5 U3" on the HPE Support Center. VMware Customer Connect (Formerly MyVMware): Navigate to the VMware Downloads page. Find the specific release for "6