hope you enjoy maths, and binary...
Otherwise this image should help you to understand :
https://dl.dropboxusercontent.com/u/57159442/photos/MnBSkillsHowTo.JPG
Skills are 4bits value (from 0 to 15) stored through 6 integers.
-First look at the right column what is the number of your skill. (if you are using a mod, then you should see this list in the mod's directory \MountBlade Warband\Modules\ModName\skills.txt)
-Then look at the middle of the image to see where the 4 bits of your skill is located and in which skill variable.
-Open your save game editor, copy and paste the skill value to this website (in "Decimal") :
http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html
-It will translate to binary, change the 4 bits pack you are interested in. You can get help from my image (the binary list on the right) to easily know what to type in.
-Copy and paste the new website value to the original value in the game save.