Your city needs to be able to be searched at accuweather. If you can then it should update fine. I opened the cab and exported the file and modified it on my PC. Then I took the file and copied it into my documents file on the 6800, if you try to copy and overwrite the file while it's in use from your pc you will get an error, then I copied it into my Windows directory with File Manager on the phone.
Your text should look like this. US cities do not require a zip code.
<city name="Boise, ID" locationcode="NAM|US|ID|BOISE" />
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