Is it possible to open the design of a game already ready to play in the unreal engine?

is it possible to open the design of a game already ready to play in the unreal engine? I wanted to modify the game ready to play.

if your talking about published games then no you cant do that and it is probably against your license / eula to do so. just think if it were a thing to modify published games how more rampant would cheating be and clones and pirates would be rampant.