What is it?
And... it's free and open source: OpenUI5 is licensed under the Apache License, Version 2.0 - see LICENSE.txt
Go to the download page and get the complete UI5 runtime and the UI5 SDK containing the documentation and many example apps as well as the complete runtime.
bower install openui5/packaged-sap.ui.core
You can get the sources and build UI5 on your own, please check the documentation for the UI5 development setup. You might then want to understand how control libraries are structured and controls are developed. Maybe there's a bug you could analyze and fix?