If you have downloaded more than one App that serves the same purpose (e.g. Skyfire, Opera, Firefox Dolphin) and selected it as the default App to perform this task and you want to change your mind to use a different App as default,
go to Menu > Settings > Applications > Manage Applications
and find the application whose defaults you want to remove.
Scroll down a bit and hit “Clear Defaults” The next time you clink a web link for example, you will get the pop up message asking you which app you would like to use to perform the task.