I accidentally "always ignore"d a wrong wrong word. How do I undo it and make the old dictionary clean again?
You can see and edit the ignored words when you open
Tools -> Check spelling ... and extend the dialog with the button "Show Ignore List":
The ignore list itself is stored in a file [language].ign alongside the dictionary files themselves. If you want to edit it directly, you should close TXS before. Otherwise your changes are not taken into account in the current session and may be overwritten on close.