How do i test customized software? (novice)

Hey, i recently bought a Youtube Clone software that allows customization and i was going to customize it myself but quickly determined it was out of my depths as a beginner who still has a lot to learn. So i decided to hire a freelancer from fiverr or upwork. My question is how can a beginner like me test the software to check that it is complete and the new features aren't buggy? Thanks and I'm sorry if this is in the wrong subreddit