Ibahka,
Take a look at the modified example. I show how to open an existing File and sheet. From your post I am not sure if you got that far or not.
Anyways you can then use Set Cell Value.vi SubVI to set the value of an 1 cell in the file and worksheet you just opened.
The example does that in a loop, thus re-writes all the data. However if you only want to set 1 cell, take the VI out of the loop, and use it independantly.
See how I changed the 0,0 cell after all the data is set.