how to change application display name phonegap

In cordova project> properties> WMAppManifest.xml file you can change you app's name and icon as well.

you actually need to go to res -> values -> strings.xml and change:

<string name="app_name">Your app's name</string>

You can set the name of your Application in your config.xml

the tag which defines it is

<name>your appname</name>

No, it is not possible due to cordova limitation:

(node:6193) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): CordovaError: The product name change ( tag) in config.xml is not supported dynamically. To change your product name, you have to remove, then add your ios platform again.