You can find the tweaker here...
http://www.delraydepot.com/tt/sh3sdk.htm
Don't forget to grab the tweak files on the same page.
The file you want to edit is called scene.dat and (IIRC) you can find it at the bottom of the data folder.
Process - open mini-tweaker, load scene.dat tweak file, make the changes as in this thread, then save new value.
Mini-tweaker creates a copy of the file you were working on (copy of scene.dat) just move this to another location to activate your changes.