I have an existing dual boot systemWindows 8 was installed first and then came Kali.Now when I'm done with Kali I want to replace it with Ubuntu.How would you suggest I do this so that from     Win8 + KaliI will end up withWin8 + Ubuntu
Ubuntu over Kali+Win8 = Ubuntu+Win8
dual boot
Couple of ways to go about it.If you are happy with the partition layout, etc. then simply install Ubuntu to the partition(s) that Kali is using, formatting during the setup process. Ubuntu will write a new version of grub that references itself and Windows, and that is it.If you aren't happy with the partition scheme that Kali is using, boot with the Ubuntu CD/DVD and delete all the Linux partitions, then use your Windows disk to fix the Windows boot loader.  Now you have a stand alone Windows machine, wiht unpartitioned space on the disk.  Boot again with the Ubuntu installer, and install it to the free space using whatever partitioning scheme you like.