Unlike their windows cousins, Linux systems are not restricted by licensing. The creators of Linux distributions, and the many applictions which run on Linux, are often distributed under a "sharing" license, a license designed so the makers of software, derivitave of the earlier software, give something back. This licensing scheme says those who make something of the "free" software sould give something back to the free software pool.
Linux and other software distributed under such licenses are called "Open Source" systems and software. Also sometimes called "Free software as in beer" and "Free Software".
Open source software, is unfettered by profit motive and related licenses, and rather is "open" so sharing of knowledge can occur, at an accellerated pace, in an unfettered way.
Upgrading a Linux system can be easier than the comperable process under MacOS or MS Windows. The Linux upgrades are easier, partly because the is no restriction due to licenses, and so the open system upgrades to not require many of the critical processes installed to protect the suppliers of Windows and MacOS software.