Note: This is for Minecraft: Java Edition specifically. This will not be applicable to other Minecraft versions.
By default, Minecraft is set to the most current version of the game. However, there are times when you may want to change to another version, such as to ensure compatibility with a certain server or mod, or to use a weekly snapshot (beta version). To change to a different version of the game, you must create a launcher configuration with the desired version, and set the game to use that configuration.
Before changing your game version, please understand that if you try to play on one of your current worlds, and you access that world using an older version of the game where certain features had not yet been added, you may irrevocably damage your world. You should back up your worlds when playing previous versions or snapshots. You can do this by locating and then making a copy of your game files.
To change the game version in the launcher you:
1. Click "Installations" on the launcher start page.
2. Click "New" to bring up the "Create new installation pop up.
3. Select a name for the insatlation and choose the game version in the "version" drop down list.
4. Press "Create" to save.
5. Press "Play" in the top menu to take you to the launcher landing page.
6. In the drop down menu to the left of the "Play" button you can now find your saved installation and choose it to play this version of the game.
Enjoy your game!