I’ve included some pointers below on how to install Ubuntu alongside PureOS, but before going through that, what software are you trying to install?
Ubuntu and PureOS are both based on Debian, so there is some common ground which means it could be possible to install the software on PureOS itself.
You can install Ubuntu next to PureOS and choose which one to use when booting. Download Ubuntu to a USB stick and boot from it to be able to install it. There are some instructions here:
You can also use GNOME Disks to create the bootable USB stick. With the USB stick plugged in, open GNOME Disks, select the USB stick in the left panel, then click the three dots at the top right, and choose “Restore Disk Image,” then select the downloaded Ubuntu .iso file.
WARNING: To actually install Ubuntu alongside PureOS, you’ll need to repartition your hard drive. Doing so could end up deleting some data from your PureOS partition, so make sure you’ve backed up everything you want before continuing with the next steps.
Reboot your computer with the USB stick still plugged in, and it should boot from there and let you install Ubuntu. If it doesn’t boot from the USB stick, then you’ll have to change the boot options in BIOS/Coreboot, depending on which model of Librem you have/whether you’ve installed Coreboot yourself.
Follow the on-screen installation instructions for Ubuntu, and which should include how to repartition your hard drive. I believe it has an automatic option for installing alongside another OS, which might be good enough for your liking, but I’d make sure to check how it’s going to partition things before actually proceeding. If you don’t like the partitioning scheme, you can do it manually.
Once you work through the instructions, you should have functional Ubuntu on your computer alongside PureOS. Reboot and choose Ubuntu when prompted to begin using it.
There are many online resources for how to dual boot, so I recommend searching if you have any questions during the process, though you can also always come back to these forums, too.