You can change keybinds in C:\Files\DayZ\[usename].dayz_preset_User.xml configuration file.
Open it with with some text editor, notepad works. (Use ctrl+f to find correct line)
Then edit the lines like this:
<input name="UAMoveForward"> [Action]
<btn name="kW" /> [Key. If you want to set it to "s", just enter "kS"]