The *.ui file is just xml so any text editor will be able to open and see the contents. But making changes to the xml with out understanding how QT will interpret it will give you endless problems. Best to just use QT designer to edit it.
Make sure you use the same version of QT designer as created the file so that you don't create incompatible properties.
Right at the top of the *.ui file you will find
in version 4 qt designer and
<UI version="3.3" stdsetdef="1">
in version 3 qt designer