Upcoming updates to RimWorld - especially mainline-updates before new DLCs release - can be accessed by opting into the unstable branch on Steam. This is mostly meant to test upcoming changes and less useful for regular play. In fact, it's highly recommend to stay on the stable branch of RimWorld - especially when using mods - otherwise game breaking errors should be expected.
The same feature can be used to switch the game version to the latest, stable patch of the previous mainline versions as well and while it's also not recommended for the average user to use anything but the latest version, we at least show you how to access this feature for whenever it is necessary to use.
To switch the game to any other provided version, just right-click RimWorld in your Steam Library and go to "Properties..." - in the menu, select "Betas" and then choose whatever version you want from the dropdown. Ludeon provides the latest, stable patch for basically every previous main version of the game - multiple selections for some - and the "unstable - Public testing" branch.
Selecting any other version from the list will have Steam to download it and replace your current install. After the download finished, you can use the game on that version until reverted.
You should always be aware that most troubleshooters in the community won't provide any support for issues with games other than the latest, stable version of the mainline branch.
In addition it is quite common for people to opt-into the unstable branch before a mainline update to test new features and then forget to opt-out again when the update actually released. Always make sure to switch back after release! The latest, stable version of the current mainline version doesn't show with its version-number, but is what shows up in the list as "None". As in; you opt-out of the 'Beta' feature altogether.