Hi i want to change my iTunes account from US store to South Korea Store and i want to reset my balance

Hello ~If you are trying to zero your balance before changing regions…you may purchase an item from the App Store for more than your balance and the remainder will be billed to your credit card on file thus zeroing your balance . You may also go to lower left hand corner and click on Support and they can zero it for you or:

Contact Apple for support and service

Also if you are changing regions take a look through this article as well:

Change your Apple ID country or region - Apple Support

~Katana-San~

—First—

Make a request to zero out any remaining balance on your account by contacting Customer Support by phone or chat:

• Chat now or call 1-800-APLCARE.
• International Support.

—Next—

Cancel your subscriptions and wait until the end of the subscription period. 

—Then—

Click ▶︎ Change Region. 🌐