Simple answer, you can't. The markdown editor is for entering markdown and the viewer is for viewing it. You can toggle the panes so you can have the editor only, viewer only or dual pane.
Another option is the richtext editor which is the button in the top right:
But you can't use actual markdown in it, you have to use the in built editor controls. The note is still saved as markdown but there are limitations.
Another option is the rich markdown plugin which is designed to make a more seamless experience and to "ditch the markdown viewer". This is probably the closest thing you are looking for?