How would you maintain legacy applications

How would you maintain the legacy applications that:

Has no unit tests have big methods
with a lot of duplicated logic have
have No separation of concern
have a lot of quick hacks and hard coded strings
have Outdated and wrong documentation
Requirements are not properly documented! This has actually resulted in disputes between the testers, developers and the clients in the past. Of course there are some non-functional requirements such as shouldn't be slow, don't clash and other business logics that are known to the application users. But beyond the most common-sense scenario and the most common-sense business workflow, there is little guidance on what should be ( or not) done.

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Solution

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