The easiest way to change the page title text (what appears in your browser bar) is to use an SEO plugin.
Just install one of those and edit the page you want to change.
If you’re using Yoast, you’ll see a new panel at the bottom of the page when you edit it.
Simply click the title and edit the text. You should add a meta description for each page also, this appears beneath the title in search engine results.