How can I change my slide from Landscape to Portait

Are you talking about Captivate, normal project? Because a responsive project has 5 breakpoint views possible, where the 3th and 5th view are normally set up for Tablet Portrait and Mobile Portrait.

For a normal project you can choose a custom size and enter the width and height in pixels (beware: browser resolution is important, not the screen resolution). But once the project resolution has been chosen, it is not appropriate to change from landscape to portrait. Maybe you should consider a responsive project.