I always forget this, so I might as well write it down. According to Apple's support pages:

Choosing another default email application

Open the Mail application (/Applications/Mail). From the Mail menu, choose Preferences. Click the General button. Choose your default email client from the pop-up menu.

Choosing another default Web browser

Open Safari (/Applications). From the Safari menu, choose Preferences. Click the General button. Choose your default internet browser from the pop-up menu.

As an extremely important side note, NEVER DELETE Safari OR Mail. You may need to change your default applications later and that is the only (easy) way to do it.

Read the full article, here.