Hold CTRL+SHIFT+C and you can open the cheat console. Enable cheats by typing testingcheats on/off. From there, you can enter the following code from the console to set the desired level on a given skill:
stats.set_skill_level [skilltype] [1-10]
As an example, you might enter stats.set_skill_level Major_Fishing 7 to set your skill level on Major Fishing to 7. The list of available skill types is
as follows: