Here is the easiest tutorial you will find on the web teaching you how to downgrade your Minecraft version!
Step 1: Watch the Video
Here is a nice video tutorial on how to Downgrade your current Minecraft version to any other version of Minecraft.
Here is the link in case you want to watch it on the YouTube watch page - How to Downgrade Minecraft To Any Version EASY (The Link will open in a New Tab)
Step 2: Open Minecraft Launcher
Open your Minecraft Launcher. (It will probably be on your desktop)
Step 3: Edit Profile
Click the little button that says "edit profile". The location of this button can vary between which version of Minecraft Launcher that you are currently using.
Step 4: Check the Boxes
Check the two boxes seen in the video. If you haven't yet watched the video, click here - How to Downgrade Minecraft To Any Version EASY TUTORIAL
Step 5: Select What Version
Now using the drop down menu under the boxes that you just checked you will be able to select the version of Minecraft that you would like to use. You can always go back to the version that you are currently using.
Step 6: Play Minecraft!
You should now be able to play the Minecraft version that you selected!