function.php in child themes

Hi

Complete novice here!! So apologies for dumb questions.

I need to change something in the functions.php file and have read lots of great advice on doing this in a child theme.

I understand that you just need to add any extra functions to the file and not copy and paste the whole original functions file into the child theme.???

But I need to replace certain code with new code and also delete one part as well. How do I do this if I don’t copy and paste the original functions file into the child theme?

I hope I am able to be understood, thank you for any help