Most mobile browsers dont give you the option to change this at all.
The Dolphin internet tool on Android is one of the few that does.
In the bottom-right corner of the popup pane, tap the cogwheel icon to fire up the prefs.
Tap the cogwheel icon in the bottom-right of the popup pane to initiate the options.
Tap User agent at the top of the Customize sub-section of the Advanced tab.
Select one of the built-in user agents or enter a custom one, then tap OK to save.
