How to alter the uc_address:city form field?
If you are using Addressfield module you can easily do that extending the module itself.

Look into the sites/all/modules/addressfield/plugin/format &amp; you will see address.inc, where you can extend the list of region or city.

You can look for the example in sites/all/modules/addressfield/example/plugin/format to better understanding of the the format which module will accept for extending it for your list values.