How to change Java-based configuration in tests for Spring-boot based application?

I’m trying to figure out how to test my code in a proper way and stuck somewhere in the middle.

What I use:

Spring-boot based Java web-application. Java based configuration. Test.

What I want:

I want to manage my testing in a proper way. I have a @RestController annotated class what has two objects what I’d like to change in tests. It’s an entities provider class BooksService and database configuration class DBConfig. I want to change implementations for these classes in my tests in a simple and proper way. I don’t know how to do it correctly.

What kind of help I need:

• Working project that implements this approach is most preferable. Don’t spend too many words, the working code is the best way to understand it for me.
• If there is no a complete project there, so code snippets with descriptions will be useful too.
• Links to some good and simple tutorials about 1) Java based configurations 2) Replacing Java based configuration for tests.

Sorry if my thoughts a little bit messy, I’m pretty new in Java EE development and still didn’t figure out with all basic topics well.