Debian 9 comes with LibreOffice (my LibreOffice is 126.96.36.199). LibreOffice, however, is already on version 6.3.0. Why doesn't it update automatically, since they're two of the biggest open source projects around, and since LibreOffice comes as default with Debian?
If LibreOffice 6.1.5 is good enough, you can either upgrade to Debian 10 (buster) or add the stretch backports repository to your apt sources. e.g., with:
echo "deb http://deb.debian.org/debian stretch-backports main" | \ sudo tee /etc/apt/sources.list.d/stretch-backports.list
I'm not sure if 6.3.0 or later will ever be backported to stretch, but it is already in the backports repo for buster. You'd have to upgrade to buster, then add the buster backports repo.
echo "deb http://deb.debian.org/debian buster-backports main" | \ sudo tee /etc/apt/sources.list.d/buster-backports.list
You could install LO 6.3.0 from the LibreOffice web site's download page, but if you do that you'll have to manually upgrade it for every new release - you'll be taking it out of debian's package management. You'd also have to purge all debian LO packages first, to ensure there's no conflict with debian's LO packages and non-debian LO.
Doing this is one of several ways that users can break their debian system.
This is not recommended unless you absolutely require version LO 6.3.0 and no other version will do, and you can't wait a few weeks or however long it takes to get into backports.
BTW, you can check which versions of LO are available for various debian releases and pre-releases and backports using Debian's package tracker:
Assuming you're using the stable release of Debian 9, its packages are only updated if major security or usability fixes are incorporated. The limitations on updating are intended to provide better stability.
Debian 9 is the version referred to as "stretch". According to https://packages.debian.org/stretch/allpackages, it includes
libreoffice (1:5.2.7-1+deb9u10) [security]. LibreOffice isn't listed in https://packages.debian.org/stretch-updates/allpackages, so there are no further LibreOffice updates in Debian 9.
If you want the latest version of LibreOffice in Debian 9, you'll need to download and install from https://www.libreoffice.org/.