Tony, I suggest using S3d - which can be found here:
http://www.subsim.com/radioroom/showthread.php?t=119571
Just open the appropriate .sim file for the boat you want to edit. I haven't looked to see what fields would need to be changed in S3d - been using it for other stuff
The other tool you can use it NvDrifter's great MiniTweaker. It (with tweakfiles) can be found here:
http://www.subsim.com/radioroom/showthread.php?t=122997
If using the Minitweaker - open the relevant .sim file and under either diesel or electric propulsion (depending on which your wanting to change) - just modify the MilesSurfaced/MilesSubmerged to equal what you want the max range to be, then set knotsSurfaced/knotsSubmerged to be the "most" economical speed - this speed will go the max distance - the miles specified above it. Any other speed will give you less distance.
Dont forget to make backup copies of files before ya edit! Enjoy and good luck!