How to modify previous line in REPL - scala to modify the typing errors to save time compare to entering each line using up/down arrows

I don't know if there is a better solution, but in my case I tend to use my usual text editor to write code snippet and paste them using the paste mode of the REPL (you can enter the paste mode thanks to the :paste command).

Right now, you can't. There are Scala GUI REPL's (see them here, plus kojo and the big IDEs), though, which allow this kind of thing. Pick one of them.