Just check your PHP installation directory for pharcommand.phar.

To see help, just type:
phar.phar.bat help
or
"php.exe" "pharcommand.phar" help

Extraction:

Syntax:

extract    Extract a PHAR package to a directory.

           Required arguments:

           -f     Specifies the phar file to work on.

           Optional arguments:

           -i    Specifies a regular expression for input files.
           -x    Regular expression for input files to exclude.
           ...          Directory to extract to (defaults to '.').

So, you need to extract the PHAR using a command like this:
"C:\xampp\php\php.exe" "C:\xampp\php\pharcommand.phar" extract -f "D:\DEV\guzzle.phar" "D:\DEV\Guzzle"

You can use relative paths if you are already in that directory.
But I find it best to give full directory paths to ensure the file gets extracted in the right folder: “D:\DEV\Guzzle”

Then suppose you change a file:
Guzzle\GuzzleHttp\Cookie\CookieJar.php

Re-pack:

Syntax:

pack       Pack files into a PHAR archive.
           When using -s , then the stub file is being excluded from the
           list of input files/dirs.To create an archive that contains PEAR class
           PHP_Archive then point -p argument to PHP/Archive.php.

           Required arguments:

           -f     Specifies the phar file to work on.
           ...          Any number of input files and directories. If -i is in
                        use then ONLY files and matching the given regular
                        expression are being packed. If -x is given then files
                        matching that regular expression are NOT being packed.

It also has a whole bunch of optional arguments that you might find useful.

After changes just re-pack the changed file using:

cd "D:\DEV\Guzzle"
"C:\xampp\php\php.exe" "C:\xampp\php\pharcommand.phar" pack -f "D:\DEV\guzzle.phar" "GuzzleHttp\Cookie\CookieJar.php"

You can even open up a phar file in a text editor like Notepad++ just to see if the changes were done successfully.
Just don’t save the phar file using the text editor because then it will stop working.