Use 7-Zip to open the new .iso, and simply drag the kernel, initrd.img (and ramdisk.img if on Pie) to the install folder, overwriting the old ones, then in 7-Zip, double click on the system.sfs file and drag the system.img inside over to replace your existing system.img. If you only have a system.sfs file, replace that with the system.img, and delete the old .sfs file. Then reboot and boot into your newly updated release