You can go to the app.UIFigure and change the identifiers. The image should be in the same folder as the app or the path must added to Matlab path
Alternatively, you can add a Callback function to the app itself like follows: 
classdef app1 < matlab.apps.AppBase
    properties (Access = public)
        UIFigure  matlab.ui.Figure
    methods (Access = private)
            app.UIFigure.Name = 'Your title';
            app.UIFigure.Icon = "C:\path\to\your\image\smiley.jpg";
And there you go: