There is a good documentation on installing PHPUnit you may follow this link

But if you don’t understand there instruction then follow my steps.

Step 1

First you need to install Composer to do this go here

Step 2

Step 3

Now need to set environment variable path to do this go to your Control Panel -> System

Then click on the Advanced system settings

It will open a new dialog box then click on Environment variables

It will open a another dialog box then bubble click on Path form System variables list

Then it will open Edit environment variables popup, then click new and add C:\bin and click Ok button. Also click Ok button on others dialog boxes.

Step 4

Go to C:\bin and open Command Propmt by press Shift key and Right button of mouse together.

It is PowerShell in my PC may be you will find Command Propmt.
Now paste this two commands

C:\bin> echo @php "%~dp0phpunit.phar" %* > phpunit.cmd
C:\bin> exit

Now open a new Command Propmt and type phpunit --version then you will find a text like below and Done!