How to change my website theme and costumization witout changing current one

i want to change my website theme so i need to do it witout losing any clients i need to make it and costumize new theme and then all the fixed posts and confugirations that i can add to current one very fast

i found lot of methods in web but i need easyone and to not harm my current one while moving to new theme

Add a child theme to override the current one.