Step 1: Start a new game
Step 2: Push U
Step 3: Esc > Options > Controls > Mouse > Turn on the Invert thing

Congratulations now you can move your mouse up without going forward!

Free look/aim mouse...I for one can't live without it....

ps: took me hours to figure all had to do was push U .. U_U

Oh don't forget to rebind the keys to WASD for convenience.

I don't think there is anyone on the forum (or anyone who plays any Build games) who doesn't know this. Since it seems like you just found out, I advise you check this thread out to make life even easier, http://www.gog.com/forum/blood_series/bmouse_0_5_mouse_driver_that_fixes_the_terrible_mouselook_on_dos_build_games.