1. make a copy of the file civ3mod.bic and back that file up, name the copy something like default_civ3mod.bic
2. Run Civ3Edit and open civ3mod.bic in that program.
3. Goto Rules--->Edit Rules and then click on the Units tab. Marine and Paratrooper are in the 5th and 6th unit down. Then just change whatever ratings you want.
I've made my paratroopers use all terrain as roads, since they are a light infantry, and I've made my marines to have 2 movement and Blitz, to better emphasize their assault role.
The easy part is just changing the numbers, the hardest, is making sure you don't unbalance the game and/or make a super-unit.
If you have PTW, then I don't know what the .bic (or .bix) file is for the basic PTW game, sorry.