First do as in the other answer and go to the Search Engines editor. The quickest way is to Right-click on the Search Bar.
You cannot, however edit the default "google.com.xx" entry since the URL part is grayed out. It looks like this:
{google:baseURL}search?{google:RLZ}{google:acceptedSuggestion}{google:originalQueryForSuggestion}{google:searchFieldtrialParameter}{google:instantFieldTrialGroupParameter}sourceid=chrome&ie={inputEncoding}&q=%s
What you can do is to create a new entry below, call it "My Google" or something, give it any keyword, and enter as the URL string the same string, substituting the {google:baseURL} part with
google.com or whichever domain of Google you want.
www.google.com/search?{google:RLZ}{google:acceptedSuggestion}{google:originalQueryForSuggestion}{google:searchFieldtrialParameter}{google:instantFieldTrialGroupParameter}sourceid=chrome&ie={inputEncoding}&q=%s
Then mouse over it, and hit "Make Default".
(Google changes this every so often on Chrome, so you probably want to grab the latest on the original/built-in search entry, and just update the google:baseURL part).
http://superuser.com/questions/293916/how-to-make-google-chrome-search-using-google-com-rather-than-google-com-hk