I think you are talking about doing some "partitioning", this is easy when you have Windows installed and do a "install along side" known as dual booting. Kind of more challenging doing it the other way around as Windows does not recognize the file systems Ubuntu uses by default known as ext2 or ext4 (not sure of Lubuntu defaults as it is either or).
You would have to use an application called gparted to create a separate ntfs partition for Windows to install on and then edit the grub2 boot loader to handle the boot process for Windows.
Honestly if you are not experienced with this and I would recommend backing up your files anyway I would do a fresh install of Windows 10 and then install Lubuntu. In the installer for Lubuntu there is a option to install along side Windows which will handle your partitioning for you and when complete and reboot grub2 each time will ask whether you want to boot into Windows or Lubuntu.
If you are looking for a good way to backup then use the "Backup" utility which I believe is installed on all Ubuntu flavors, if it is not on Lubuntu then run this command.
sudo apt install deja-dup
Then you will have the backup utility installed.