Boot QEMU pointing to your Windows 10 ISO and the new QCOW2 file as the primary drive. Complete the installation as normal.
Shut down the VM and use the qemu-img commit command if you are using a backing file. Method B: Injecting Updates (Offline)
The simplest way to update your QCOW2 image is to boot the virtual machine and use the built-in Windows Update utility: Start the VM in your hypervisor. Go to Settings > Update & Security > Windows Update. Click "Check for updates" and allow the process to finish.