If you want to use raster calculator, try this:
meaning that if raster1 has no value, it will use the value of raster2, otherwise it will use the value of raster1. Under the
Enviroments... tab you can select your desired extent of the output raster. This will only work, if your raster1 has a lot of gaps.
However, I guess the best solution would be to use the
mosaic to new raster tool in your
Data Management toolbox, since you only want to replace values and not perform any other operation. Look at the following example:
Make sure the
Mosaic operator is set to your raster2, so that values from it will be used instead of raster1.