I have an alternator that only charges the batt at around 2000rpm. At idle my volt gauge reads 12V and at 2000rpm it reads 14-14.4V.

It has a really big pulley, but if i change to a smaller pulley will it fix it so it charges at idle? Or will it just blow the alternator and i should replace it?

 

bump de de bump

 

Alternators are not efficient at idle. Putting a large pulley on, will make it less efficient. Many hot rodders recommend that you Do Not put an underdrive pulley on the alt. for this reason.
Yes, replacing the over size pulley with a smaller one(stock) will help with your low rpm charging.