How to add code in <head> of WP page

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I worked on a two-year contract \[’14-’16\] updating a customer’s WP site and haven’t used it since. Now I’m with a new company and need to start using WP again.

Suppose I want to put some HTML on a page, I know I can do this with the “text” tab. But suppose I need some CSS or whatever for this, is there a way to put the CSS in the <head> without having to go onto the server and find the page and do it there?