There's also a stand-alone designer you can use on individual ui files. In general, there's no need to have the whole project in Qt Creator just to edit a ui file.

You can also edit the ui files in a normal text editor. They're just XML files that specify the widget hierarchy to construct. If you have a few example files to work from, they're pretty straight-forward to understand and modify. Validating your changes using designer is definitely faster than recompiling your program, but it's an option.