Back in the early days of OS X, Apple’s desktop operating system shipped with an Internet preference pane that let you change, among other things, your default Web browser and email app. At some point, Apple decided to put these settings in Safari and Mail, respectively, but with OS X Yosemite and later, the option to change your default Web browser returned to its rightful home in System Preferences.
If you’d like to change your Mac’s default browser, open System Preferences (look in the Apple menu if you don’t know where to find it), then click General. Next, find the pop-up menu labelled “Default web browser:” Click it, then choose whichever browser you’d like to use as your default.
There are a couple other things you’ll want to be aware of. First, the Default web browser menu lists any app on your computer that can open Web pages, even if they aren’t necessarily a Web browser, per se. On my computer, for example, Evernote, Cyberduck (an FTP player), and Flip Player (which brings Windows Media compatibility to QuickTime) appear in the pop-up menu, but none of those are browsers.
Second, if you want to change your email client, you still need to do so through Apple’s Mail app. Open Mail, then choose Preferences… from the Mail menu. Click General, then select a new email app from the Default email reader pop-up menu.