If you wish to decode to GeoTIFF directly from the GeoExpress graphical user interface, you can easily do so. Simply choose the GeoTIFF output format and encode as usual.
To decode to GeoTIFF (.tif) or to JPEG (.jpg) you can also use LizardTech’s free command-line utility, mrsidgeodecode, which is installed alongside GeoExpress and can be found in its “bin” directory.
Windows users can also use LizardTech’s free GeoViewer to view MrSID images and decode them to GeoTIFF, JPEG or PNG (.png) format.