There is no way to change the default browser of your iDevice… unless you Jailbreak it.
It’s a bit strange and uncompetitive that Apple have yet to introduce a setting in iOS that allows you choose your default app for things such as browser, media, and mail. I should be able to choose Opera over Safari on iOS in the same way that I can choose Opera as my default browser on OS X.
If your device is Jailbroken you can install the Browser Changer app for free from Cydia (within the ModMyi repository). This will allow you to override Mobile Safari and choose an alternative browser such as Opera Mini, Atomic Web Browser, or Skyfire Web Browser instead.
(Via App Advice)