1. Go the Unity Download Archive Here
2. Once on the Download Archive page, scroll to Unity 5.3.5
3. If you are using a Windows system, click on the Downloads (Win) button, and on the drop-down menu, choose Unity Installer.
4. If you are using a Mac system, click on the Downloads (Mac) button, and on the drop-down menu, choose Unity Installer.
When Unity has finished downloading, click Run to start the installation.
1. When the Unity Installer window opens, click Next
2. After that, accept the terms and conditions and click Next
3. You will then see a screen called Choose Components. You will need to choose the following components from the list:
The top four components should be ticked already, and WebGL Build Support is right at the bottom of the list:
4. After selecting these, click Next and choose your installation location.
This will start the download and installation process. Once finished, Unity will be installed.