Redirect, without losing pages ranked

I want to redirect my wordpress pages without losing the pages ranked in Google
I know it is possible because I have seen people doing it, but what script?

It's not really a script, it's called 301 redirect. Several types of redirect exist but 301 is the best one to keep your rankings. You can use a plugin like this one to make the job easier: https://wordpress.org/plugins/simple-301-redirects/ (there are many others, just pick up the one you like)