Changing your Google Store Code in Dealer Kit is a straightforward task:
You need to click on your name once you're logged on to Dealer Kit and then Settings in the top right corner.
Scroll down and find the location that you want to change your store code for click the pencil icon.
Click the Edit button again in the top right hand corner, scroll down to the bottom and fill in the Google Store Code box and click Save
Your Store Code is now set correctly. Next time the feed is pulled to the Merchant Center, the Store Code will be reflected.