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 an old alpha or beta version. The steps below will explain exactly what you need to do to play such a version.
Open the Minecraft launcher and click Installations.
Enable alpha/beta versions by checking the historical checkbox.
Create a new version by clicking New.
You can now select an old alpha/beta version. In the version menu, scroll all the way down until you find yourself the beta versions. Scroll even further down and you’ll see alpha versions.
Hit Create to create your alpha/beta version. Before you do this, you might want to name your version so it’s easily recognizable.
Go back to the play menu and select your alpha or beta version from the version menu at the bottom of the page. In this case, old beta 1.2. Then hit play to play this version.