Although it's very concerning to show a foreign country flag on your campaign form, the fix is actually very easy.

First, just go to your Campaigns list and edit the Campaign that you wish to change the flag on its form.

Then right on Step 1, choose the country that you desire to change the flag to. 

The flag icon in the phone number field will represent the country that you choose in that step, and will auto-generate the country phone number code as well (i.e. US = +1, UK = +44, etc.)