When beef is turning brown it means that it's losing it's freshness and starting to go bad.
Usually beef that is turning brown just means it's not as fresh as before and most times it hasn't spoiled yet and is still safe to eat.
As long as the beef isn't slimy or has a foul smell to the beef then it's perfectly fine to eat.
But if the brown beef has any slimy feeling to it or it smells bad then it's unsafe to eat and should be thrown out.
If it's bad and has a bad smell to it you'll certainly be able to smell the bad smell that will let you know that it's no longer safe to eat.