Before uploading a new terrain file you should download the existing one and keep it safe. This gives you the option to revert back to the old one.
Open up the Second Life application, login and select Region/Estate from the World menu at the top of the screen. Select the Terrain tab and then click the Download RAW Terrain button. When prompted, save the file in a safe place
Open up the Second Life application, login and select Region/Estate from the World menu at the top of the screen. Select the Terrain tab and then click the Upload RAW Terrain button. Select your new terrain file.
Baking your terrain produces the equivalent of an undo feature. It allows you to revert any changes back to the last time the terrain was baked.
Open up the Second Life application, login and select Region/Estate from the World menu at the top of the screen. Select the Terrain tab and then click the Bake Terrain button.