immutable strings in c# - how to replace a single character in a specific position

So strings in c# are immutable. 
while I can read a character, I can't write back to it. 
for (int i = 0; i < queryOut.Length; i++) { char c = queryOut[i]; <------ this is OK .... rest of my code queryOut[i] = '$' <----- this is NOT OK. }
How can I replace a specific character in a string knowing it's location?
I can't use the .Replace() method because that replaces everything. I just need to replace certain character in a known index position.