Apple doesn’t make it easy to change the default web browser on an iPad, iPhone, or iPod touch. While you can install plenty of alternate web browsers, any time you click a link in an email, text message, or other app, it will open in Safari.
But if you have a jailbroken device, you can install a third party app called BrowserChooser which lets you change the default browser.
Right now BrowserChooser supports a number of web browsers including Google Chrome, Dolphin Browser, Opera Mini, and a number of others… as well as Safari, which means you can always switch back to Apple’s browser.
The iMore blog has instructions for installing BrowserChooser on a jailbroken iOS device.