The easiest way to get a development version is with composer require flashbackzoo/silverstripe-charts --prefer-source
Don't forget to run the tests ./vendor/bin/phpunit silverstripe-charts/tests/
To update the front-end (JavaScript, CSS) you first need to install the dependencies with npm install
Once you've don that, run npm run build to compile the JavaScript.
The main JavaScript for the module is static/js/src/main.js
You can watch for changes with npm run build:watch