Edit HTML of Landing Page

Unfortunately you can't do that within the landing page editor. 

Depending on what you are trying to achieve, you can add a Custom HTML Block to the page and add some jQuery to it to push items around on the page or apply certain styling to them.

Within the Landing Page Designer, you can add custom HTML sections and Rich Text sections among other types of sections.  You can edit the Rich Text section using the Rich Text Editor, which has an HTML button in the toolbar that allows you to edit the HTML version of the text.  You can use the Custom HTML Editor to edit the custom HTML on the landing page.  You can edit some of the template elements upon which the landing page is based by dropping down the Property Sheet at the top of the page and selecting the desired template element at the bottom right pane.

I agree wtih Elliott. Use the HTML section to drop HTML into your landing page.