Changing Payment Gateway to US

[Ecomcommando]

Hi, I have had many abandoned checkouts as customers in the US were getting errors such as "This card cannot be used for this purchased" or "Your card was declined" and there did not seem to be an issue with their funds. My store has two locations (US and Australia) but i have changed the default location to US now. What i want to know is how can i change the payment gateway to US to avoid getting card declines for my customers that are occurring quite frequently now.