You can edit the html in text blocks within the editor by selecting your text block, then clicking the Source editor button in the properties panel on the left as shown below:
The source editor will open in a seperate window allowing you to edit the html for the selected block
For image blocks you can edit the Link, Alt text and the Image source URL in the properties panel on the left side of the editor as shown below:
You can not edit HTML for the entire newsletter structure, only for specific blocks. If you want control over the HTML of your entire newsletter please use the Code custom HTML option to create your newsletter.