There are 2 possibilities as far as i know.
Use the console:
Bring up the console and type
RESETSKILLS to reset all your skills.
Remember however, this may flag your account as suspicious in Multiplayer!
How to enable the console in Torchlight II
Overview of all the commands in Torchlight II
Use a shared stash savefile:
There's a shared stash savefile here, to get potions that let you reset skillpoints. This will not flag you as a cheater, but is a little different.
First, quit the game and turn off Steam Cloud.
Then, navigate to your Torchlight II save folder (typically:
C:\Users\Username\Documents\My Games\Runic Games\Torchlight 2\save\RandomNumbers) and make a copy of (or just rename) the sharedstash.bin file.
Now extract the sharedstash.bin file of the above link in the save folder.
You can now launch the game again and in your
Shared Stash there will be some potions. Using a potion resets all your skillpoints.
Finally, to get your original shared stash back (not necessary if it was empty at first), you quit the game again, remove the downloaded sharedstash.bin file and replace it with the original sharedstash file.