why can’t you simply upgrade to latest version in ubuntu
You can (perhaps depending on what version of Ubuntu you are running), either by using a PPA, such as https://launchpad.net/~libreoffice/+archive/ubuntu/ppa and upgrading through your usual package manager, or by downloading the DEB version and following the installation instructions in the README file.
1) Knowing how it is, forget about upgrading/updating to newer packages from the official repos, especially when the program gets a new version quite often, like LO. That’s why the official PPA (https://launchpad.net/~libreoffice/+archive/ubuntu/ppa) with dedicated ppl (who can be busy with something else, too, but that’s considerably rare) exists.
That’s true for most programs and games you can install on an Ubuntu, but whether some PPA providing newer versions supports the distro version, or worse yet, whether it works, can be different story. If the release isn’t supported, you can still try to install from it by manually adding the repo, and what you need to know is always listed in the PPA’s page right there, hidden behind single click.
2) Also be warned you might not be able to install or use a DEB package in either Ubuntu or Debian due to format-breaking changes Ubuntu made; a Debian DEB should work in Ubuntu but an Ubuntu DEB might not work in Debian. Anyway, you can always try if no repo works.