How do I change the title and description Google shows?

In the editor press the Settings cog, open "This page" → "Search & sharing" and fill in Title and Description. It previews how the page looks on Google and when shared.

Leave them empty and we use the page name with your site name, and the first paragraph on the page. Tick Hide this page from search engines for a page you do not want listed.

Site-wide defaults — tagline, description and share image — are under the Site tab.