First, fork this repository, checkout it locally and then install project dependencies with Composer - composer install.
Now make all your changes and test them.
To test the changes you will obviously need a bot, assuming you already have one - put the token and bot username in .env file for local development.
The easiest way to test your changes is to run the bot with getUpdates method - use php bin/console loop command.
Make sure your code is following PSR-2 coding standard - run PHP_CodeSniffer with composer check-code command.
Now when all seems to be good push you changes to a new branch in your fork and then create a pull request explaining all the changes.