Install Unity :

Below are the steps to install Unity 3d:

1. First download the Unity 3d from the URL: [http://unity3d.com/unity/download][1]

2. Then double click on the UnitySetup-4.3.1.exe

3. Click on Next button to set the installation path.

4. After setting the path click on the Install button.

5. Then Finish.

Installing Multiple Versions of Unity:

You can install more than one version of Unity on your machine as long as you follow the correct naming conventions for your folders. You need to rename each of the Unity folders themselves, so that the hierarchy looks like:

Unity_3.4.0

---Editor

---MonoDevelop

Unity_4.0b7

---Editor

---MonoDevelop

Below are the steps: 1. Install Unity 4.0

1. When you install on PC it will select the previously installed directory - do not install here.

2. Create a new directory named sensibly e.g. Unity_4

3. Name any shortcuts so you know which version you are launching.

4. Hold Alt when you launch the beta to force unity to let you choose which project to open (otherwise it will try and upgrade the last opened project)

5. Choose your projectname_4 directory to open your backed up project

Note: Do not rename each Editor folder inside a single Unity folder! You will overwrite the MonoDevelop folder and this will cause serious stability problems and unexpected crashes.