Question - what do you want to replace it with? Also, how do you want the replacement text to be formatted?
Without knowing more it is difficult to give you a complete answer. So assuming you are not concerned with formatting:
What I would do is draw a text frame and type the word Sünder (with the umlaut over the ü), copy it, and paste it into the Find field.
Make sure all formatting settings in Find/Change are cleared. You should be able to do a text search - GREP is not necessary, but that depends on what you are replacing the word with.
Click "Find" and see if it finds the word for you.
Put what you want to replace the word with in the Replace box.
See if that works for you. But if you can be more specific, I can give you a more detailed answer.