iOS NOT READY YET!
To set a new UserAgent add the following to your config.xml
<preference name="useragent-android" value="My new UserAgent" />
<preference name="useragent-ios" value="My new UserAgent" />
To append the existing UserAgent add this to config.xml
<preference name="useragent-android-append" value="-Appended_string" />
<preference name="useragent-ios-append" value="-Appended_string" />