1. Create a new Qt Widgets Application.
2. Press Ctrl+Shift+F to open an Advanced Search.
3. Select the current project as search scope, type "300" as search phrase and click "Search & Replace".
   A line in the .ui-file is being found:

   <height>300</height>
   

4. Double-click this search result.
   The .ui-file opens in Edit mode.
5. Type a different number into the "Replace by"-Edit and click "Replace".
   300 is replaced by the number you typed.
6. Close the file.
7. Open the file again.
   "300" is back again. The file was not changed.

"Search & Replace" should save its changes.