To change cities you just need to create a new ad and delete or pause the old one. This takes less than a minute!
Go to your Ads page (Ads tab at the top of your dashboard. Select "Pause Ad" or "Delete Ad" on the old ad, then click the "+New Ad" button to create a new ad in a different city:
Select "Pause" or "Delete"
Click the blue button "New Ad" to create your ad:
Be sure to also change your current location by clicking on the black 'Currently in..' button located at the top of your main dashboard page: