If you have a multiboot installation and want to install in another partition of your PC instead of copying the ISO to a USB disk you can load the ISO into a folder on your disk and use grml-rescueboot:
Install with sudo apt install grml-rescueboot
Put the ISO on your disk /boot/grml/ do update-grub and at next boot you will find an entry in your grub menu.
You may also edit /etc/default/grml-rescueboot to use your Download folder:
## Configuration file for Debian package grml-rescueboot.
# Location of ISOs:
# ISO_LOCATION="/boot/grml/"
ISO_LOCATION="/home/corrado/Downloads/"
....
You may put many ISO in your folder and select from grub menu
corrado@corrado-n3-qq-0918:~$ sudo update-grub
…
Found Ubuntu 24.04.2 LTS (24.04) on /dev/sda2
Adding boot menu entry for UEFI Firmware Settings …
Found grml-rescueboot ISO image: /home/corrado/Downloads//qq-0926.iso
done
corrado@corrado-n3-qq-0918:~$
Here an example of output from update-grub with 2 images in my Download folder.
To have various images in a folder i rename them adding a date. The image may be renamed without problem.
I successfully installed from home/Downloads many ISOs of Ubuntu recent versions and also Xubuntu, Lubuntu and Debian.
corrado@corrado-n8-qq-0926:~$ sudo update-grub
[sudo: authenticate] Password:
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/kdump-tools.cfg'
Generating grub configuration file ...
Found background image: gorilla2.png
Found linux image: /boot/vmlinuz-6.17.0-5-generic
Found initrd image: /boot/initrd.img-6.17.0-5-generic
Found memtest86+ 64bit EFI image: /boot/memtest86+x64.efi
Found memtest86+ 32bit EFI image: /boot/memtest86+ia32.efi
Found memtest86+ 64bit image: /boot/memtest86+x64.bin
Found memtest86+ 32bit image: /boot/memtest86+ia32.bin
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
Found Ubuntu 25.04 (25.04) on /dev/nvme0n1p2
Found Ubuntu Questing Quokka (development branch) (25.10) on /dev/nvme0n1p3
Found Ubuntu 24.04.3 LTS (24.04) on /dev/nvme0n1p4
Found Debian GNU/Linux forky/sid on /dev/nvme0n1p7
Found Ubuntu Questing Quokka (development branch) (25.10) on /dev/nvme0n1p9
Found Ubuntu 24.04.1 LTS (24.04) on /dev/sda1
Found Ubuntu 24.04.2 LTS (24.04) on /dev/sda2
Adding boot menu entry for UEFI Firmware Settings ...
Found grml-rescueboot ISO image: /home/corrado/Downloads//qq-daily-0905.iso
Found grml-rescueboot ISO image: /home/corrado/Downloads//qq-daily-0926.iso
done
corrado@corrado-n8-qq-0926:~$