How to modify the content of text file

[grsandeep85]

Hi All,

i have a text file in the format
#n X Y Z
0 123 456 789
1 321 654 987
2 345 243 809
......
.....
...... and so on

the user inputs a linenumber of the text file from the number lineEdit the corresponding X,Y,Z values of that line i am able to read and put into the corresponding lineedits.now i need to modify the contents of linenumber and again write into the same file, please help us how to do this.