Wait build to be processed and update changelog afterwards. fastlane 2.125+ required.

update_tf_data.rb

require 'spaceship'

module Fastlane
  module Actions
    module SharedValues
      UPDATE_TF_CHANGELOG_CUSTOM_VALUE = :UPDATE_TF_CHANGELOG_CUSTOM_VALUE
    end

    class UpdateTfDataAction < Action
      def self.run(params)
        # fastlane will take care of reading in the parameter and fetching the environment variable:
        UI.message "Parameter app version: #{params[:app_version]}"
        UI.message "Parameter build version: #{params[:build_version]}"
        UI.message "Parameter changelog: #{params[:changelog].to_json}"
        
        app_version = params[:app_version]
        build_version = params[:build_version]
        
        
        app_identifier = CredentialsManager::AppfileConfig.try_fetch_value(:app_identifier)
        team_id = CredentialsManager::AppfileConfig.try_fetch_value(:itc_team_id)
        
        Spaceship::Tunes.login()
        Spaceship::Tunes.select_team(team_id: team_id)
        app = Spaceship::ConnectAPI::App.find("com.facemetrics.reader.ios") 

        @build = FastlaneCore::BuildWatcher.wait_for_build_processing_to_be_complete(app_id: app.id, app_version: app_version, build_version: build_version, return_spaceship_testflight_build: false)

        puts @build
        puts @build.beta_app_review_submission
        @client = @build.client
        localization = @build.get_beta_build_localizations.detect{|b| b.locale == "en-US" }
        
        if localization != nil 
          @client.patch_beta_build_localizations(localization_id: localization.id, attributes: {"whatsNew": params[:changelog]})
        end

        UI.message("Your build #{@build} sucessfully processed")

      end

      #####################################################
      # @!group Documentation
      #####################################################

      def self.description
        "A short description with <= 80 characters of what this action does"
      end

      def self.details
        # Optional:
        # this is your chance to provide a more detailed description of this action
        "You can use this action to do cool things..."
      end

      def self.available_options
        # Define all options your action supports. 
        
        # Below a few examples
        [
          FastlaneCore::ConfigItem.new(key: :app_version,
                                       env_name: "FL_UPDATE_TF_DATA_APP_VERSION",
                                       description: "Application version to wait for"),
          FastlaneCore::ConfigItem.new(key: :build_version,
                                       env_name: "FL_UPDATE_TF_DATA_BUILD_VERSION",
                                       description: "Build version to wait for"),
          FastlaneCore::ConfigItem.new(key: :changelog,
                                       env_name: "FL_UPDATE_TF_DATA_CHANGELOG",
                                       description: "Update changelog for concrete Testflight build")
        ]
      end

      def self.output
        # Define the shared values you are going to provide
        # Example
        [
          ['UPDATE_TF_CHANGELOG_CUSTOM_VALUE', 'A description of what this value contains']
        ]
      end

      def self.return_value
        # If your method provides a return value, you can describe here what it does
      end

      def self.authors
        # So no one will ever forget your contribution to fastlane :) You are awesome btw!
        ["github/OdNairy"]
      end

      def self.is_supported?(platform)
        platform == :ios
      end
    end
  end
end

Updated action to support latest ConnectAPI methods. Requires fastlane 2.125+