With TYPchanger you can change the layout of a map on your device by adding another TYP file to the map file. The TYP file has to be compatible with the map; you can't just use any TYP file.
You can also use GMapTool to do this (actually, TYPchanger uses GMapTool), but that's a fairly complicated program, and it requires multiple operations. The idea behind TYPchanger is to make it as simple as possible.
If you want to change or add a TYP file to a map on your computer, then use JaVaWa GMTK.
Using TYPchanger is simple: run the program, drag the map file (with the extension .img) to the window, do the same with the new TYP file and click "Change".
Unfortunately there is no Mac version of this program because the Mac version of GMapTool doesn't support adding TYP files.
Fixed problem with upper-case filename extensions.
Added check for name length of TYP file.
You can also very easily replace the TYP file via ImgTool: