That's easy, just press "U" in the game, which I think is the default key. Under "Options -> Controls -> Configure Keys" this key can also be changed, and in the controls options you can change the mouse sensitivity and if the Y-axis should be inverted or not.
In the external setup menu you can configure the controls even more extensively. There is a shortcut in the Windows start menu under "One Unit Whole Blood [GOG.com] -> Settings", or run the setup.exe using DOSBox if you know how.
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What's more, aiming up and down in Blood (and the other Build engine games) feels rather bad. This can be fixed to a degree by using bMouse. For this you need to download it from
SwissCM's homepage here:
http://swisscm.duke4.net/mine.html Then proceed as follows (modified from the bmouse.txt):
1. Unzip this archive into the directory where you have installed Blood
2. Open the setup as described above
3. Go to "Controller Setup -> Choose Controller Type" and select "Keyboard
and External"
4. Go to "Setup External" and change the program name to "BMOUSE.EXE"
5. Then press "Save and Launch Blood" and see how you like it!
Alternatively to bmouse.exe, you can also use RWesterlund's modification of it called smouse, which is said to improve mouse control even further:
https://www.gog.com/forum/blood_series/mouse_input_lag/post6 Note that when using bmouse or smouse you can't change the mouse sensitivity in the ingame settings, only in the external setup under "Controller Setup -> Setup Advanced Control Options -> Advanced Mouse Setup" and then "Mouse X Axis Scale" and "Mouse Y Axis Scale" for the sensitivity of respectively looking right/left and up/down.
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Lastly, if you want even better, more modern (mouse) controls and don't mind that a few things might not be exactly as they are in the original DOS version, you can try BloodGDX, which is a reverse engineered port of Blood to work natively on modern computers. It is rather quick and easy to install and use, far into development and can be found here:
http://m210.duke4.net/
Just click on the latest release on the right side (Currently "BloodGDX Release v0.784") and then the button above "Download now!" on the left. Installation instructions are included in the readme.txt, usually you just have to put the "BloodGDX.jar" file in your Blood installation folder and run it.
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I think that just about covers all your options, have fun! :)
