A few ways to do it. - Use the CON tool in Spatial Analyst - use RasterToPoint, edit the values in the feature class and then do PointToRaster (remember to set the cell size and extent to that of your original raster) - RasterToASCII, edit the text file, ASCIIToRaster
Thanks. I was sort of hoping that Arc would by now allow you to select a cell, change the pixel value, and save. And not have to create an interim layer or dataset. I have certain locations where I want to extrude the cell to a certain height in the DEM.