The easiest and by far the fastest method would be to head over to Downloads and download the latest .unitypackage file. Alternatively you could unzip the source into your unity project's directory. In this guide we're going to cover installing via .unitypackage file.

First you will need to open a project in Unity. In this example we created a new project named Sys API Test

After you have your Unity project loaded, download and open the latest Sys API Unity Package in File Explorer.

Next just drag and drop the unity package file into your project. Once it decompresses click 'Import' and your done!

Now would be an ideal time to check out the Scripting Library to see all the methods available in the Sys class.

Updating the API is relatively simple. You could keep downloading the latest unity package and syncing the files, but we created an editor script to make updating even easier. The update system works via the SysUpdateDownloader and the SysOptions scripts. First the SysOptions starts play mode in the editor, then calls SysUpdateDownloader to download the vc.dxt file from here on the github. Then it checks the servers version code against the local version code. If the VC is different then the SysUpdateDownloader will download and replace the following scripts with the scripts on the server: SYS_MASTER.cs, change.log, Documentation.txt, Sys_API_CSharp_Example.cs, Sys_API_JS_Example.cs, and SystemInformationLogger.cs.

NOTE: editing or altering vc.dxt, SysOptions.cs, or SysUpdateDownloader.cs is NOT recommended and could result in errors.

You can 'Force Update' the API to bypass vc.dxt checking and re-download the same package. This is generally used if somehow the API becomes corrupted. Keep in mind that any prior script edits will be lost.

New lets go over how to download updates from within the Unity Editor. Here's a step by step guide taken from the Read Me:

In the Sys API source package is two c# scripts that handle updating the API via the unity editor. To update the Sys API navigate to ‘Tools/SysAPI/Options’ or alternatively press Control+W to open the Sys Options editor window.

You will see a new window appear that looks something like this:

The options window contains three buttons: Check For Updates; Force Update; and Example. Pressing ‘Check for Updates’ will check your version versus the latest version of the Sys API. If you have the latest version you will receive this message:

If there is an update available the newest SYS_MASTER.cs will be downloaded and saved to ‘/Assets/Plugins/SysAPI/SYS_MASTER.cs’. After updating this message will display in the console:
If somehow SYS_MASTER.cs becomes corrupted you can press the ‘Force Update’ button to grab the latest update from the server.

Lastly, pressing ‘Example’ will generate a new game object named SYS and attach Sys_API_CSharp_Example.cs and Sys_API_JS_Example.js scripts (same as the 'SYS' object in the Example.unity scene).