This is a beginners tip.
Lot of firms want the Ground level (First Level) to be 100'. Or the actual level height given by the civil Engineer. Trying to change the level directly by editing the Revit level won't work. (It might actually move the model and so, all your view crops, etc may become wrong!)
Instead, while in an Elevation view, go to Tools menu > Project position/orientation > relocate this project... and then click in the drawing area and move your cursor up and enter 100' (or whatever distance you want the levels to move)
Zoom all and view the levels, it would most probably not have changed. Select the level head and go to its type properties and change its Elevation Base to "Shared"
That should do it.