[Let's Go] [Pikachu] Caught Mewtwo and saved, how to change nature, how to swap it with a shiny

Title says all, how can I replace my Mewtwo with a shiny? Beating E5 doesn't bring him back.

If you've already caught Mewtwo, it's too late. There's typically only one of a species of a legendary Pokemon per save file.

The whole beating the Elite 4 thing with legendaries is to make them reappear if you KO'd them while trying to catch them, not to give you an endless supply of them.

Can I somehow ( on the same account ) make a trade of a shiny Mewtwo with a new save file?

that I don't know