If you want to patch up 1.4 to 1.5, go to the 3D Realms downloads site:
http://www.3drealms.com/downloads.html and scroll down to Duke Nukem 3D.
There you'll find a line which says "4.
ppack15.exe - v1.4 to v1.5 registered patch (673k)"
So download that, and run it in dosbox, which can be found here:http://www.dosbox.co...load.php?main=1 (I'm guessing you'll want to download the windows 0.74 one).
When you run dosbox it shows you some pretty self explanatory instructions so that shouldn't be a problem.
Go to the directory where you installed the patch (btw you should put your 1.4 GRP there before you do any of this) and you should have a brand spanking new duke3d.grp for version 1.5!
Note I've never actually done any of this before so if anyone else can correct me if I'm wrong about anything, that'd be helpful, but what I've said is probably the way to go.