UI or User Interface editing is done by editing .SC2Layout files. SC2Layout files are renamed XML files, and can be found in /.../StarCraft II/Mods/Core.SC2Mod/Base.SC2Data Base.SC2Data is an MPQ (File Format) file. You'll need to learn how to edit MPQ files to extract the .SC2Layout files. Inside the MPQ, they're found at UI/Layout/UI.
Extract all the files you find inside that folder into a folder on your computer. It's probably a good idea to make duplicates, and have one folder filled with the original .SC2Layout files, and one folder filled with the ones you're currently editing. The SC2Layout files can be opened with any text editor, such as Notepad, Notepad++, gedit (Linux), etc.
Here is a list of all the current .SC2Layout files included with a default StarCraft II install:
TODO: Stuff about editing them
To add your edited .SC2Layout files to your map, there are two things you can do. One is to open your map in an MPQ editor, and place the edited .SC2Layout files into a new folder located at [yourmaphere.sc2m]/UI/Layout/UI The other way is to add your .SC2Layout files by opening your map in the Editor, opening the Data Editor, and navigating to UI - Custom Layout Files at Data Editor - Game UI Data. Then add your files, and save the map.
Note: You can import your .SC2Layout files using GalaxyEdit's default Importer, but you have to save the map after everything you do. If you want to change your .SC2Layout files, you need to remove all of the imported ones, save, import the new ones, and then save again.
<youtube>8-MwK7Racv4</youtube> A tutorial by OneTwoSC about editing .SC2Layout files and importing them into your map