Unity PostProcessBuid script for editing the PList file within the Xcode project. This example will enable push notifcations.

PListiOS.cs

using UnityEngine;
using UnityEditor;
using UnityEditor.Callbacks;
using System.Collections;
#if UNITY_IOS
using UnityEditor.iOS.Xcode;
#endif
using System.IO;

// PListiOS - Edit the PList file.

public class PListiOS 
{
	#if UNITY_CLOUD_BUILD
	// This method is added in the Advanced Features Settings on UCB
	// PostBuildProcessor.OnPostprocessBuildiOS
	public static void OnPostprocessBuildiOS (string exportPath)
	{
	Debug.Log("[UCB] OnPostprocessBuildiOS");
	ProcessPostBuild(BuildTarget.iPhone,exportPath);
	}
	#endif

	[PostProcessBuild]
	public static void OnPostprocessBuild (BuildTarget buildTarget, string path)
	{
		#if !UNITY_CLOUD_BUILD
		Debug.Log ("[iOS] OnPostprocessBuild");
		ProcessPostBuild (buildTarget, path);
		#endif
	}
		
	public static void ProcessPostBuild(BuildTarget buildTarget, string path) 
	{
		#if UNITY_IOS

        if (buildTarget == BuildTarget.iOS) {

			Debug.Log ("[iOS] OnPostprocessBuild - PList");

			// Get plist
			string plistPath = path + "/Info.plist";
			PlistDocument plist = new PlistDocument();
			plist.ReadFromString(File.ReadAllText(plistPath));

			// Get root
			PlistElementDict rootDict = plist.root;

			// Change value of CFBundleVersion in Xcode plist
			var buildKey = "UIBackgroundModes";
			rootDict.CreateArray (buildKey).AddString ("remote-notification");

			// Write to file
			File.WriteAllText(plistPath, plist.WriteToString());

		}

		#endif
	}
}