You can undo when you go back to the Design tab.

Here is a post about how you can set it not to save. Next time try Google. This was the first link that came up.

http://social.msdn.microsoft.com/Forums/en-US/8af282b6-eb14-43bf-873a-cdfdf5f55311/turn-off-autosave-after-preview-in-ssrs-2008