Is this clearer? You do have ADB ("USB debugging") working? You do have fastboot installed?
Code:
C:\>adb reboot bootloader
C:\>fastboot boot old-magisk.img
C:\>adb shell
$ su
# dd if=/dev/block/by-name/boot of=/sdcard/boot.img
Use Magisk.apk to modify /sdcard/boot.img
Code:
C:\>adb pull /sdcard/new-magisk.img
C:\>adb reboot bootloader
C:\>fastboot boot new-magisk.img (just to double check)
C:\>adb reboot bootloader
C:\>fastboot flash boot new-magisk.img