Pregunta de entrevista de TuneIn

How to rewrite the older codebase into a newer codebase that doesn't have older .net framework? Older .net framework have some libraries that couldn't be found in the newer framework. The system is running all the time, so it won't be possible to take that offline. There's no testing suits developed to test the software. How to test and fix the bugs without testing suits? Do you have any experience working with that type of problem?