Blender as no builtin automatic update system at the moment, you always have to download and manually install new versions.
They are for the most part backwards compatible except for major versions like the jump between 2.49 to the 2.5 series, or between 2.79 and the 2.8 series. Apart from some small additions or changes you should always be able to open your files without problems on newer versions unless stated in the release notes.
If there is no specific functionality from the dropped you specifically need, it is always advised to upgrade, unless you are in the middle of critical project, commercial production work, or a complex scene; in which case you are advised to finish your work before messing with the software installation.
If you want something close to an auto update system you can try the Steam version of Blender.