Bildy engine is open source and uses OSL license. This means, that you're welcome to edit and refine everything you find on your server space after installation.
Bildy has been designed to be as flexible and open as possible. Modifying the Bildy Engine is not a trivial task, so we recommend you to consider other ways to solve your problem. It is often enough to create custom controls or modules.
Also please note that if you modify Bildy Engine itself, you should disable the auto update feature of Bildy Editor by setting your engine version to a high number. The version file can be found inside the bildy folder.
If you know what you're doing and want to make things better, go for it! We would be glad to hear about your modifications.
For further help, contact support@thebildy.com or get familiar with the source code.