Replacing a Character

Hi,

i need some help with this particular task.And the task is as follows:

I need to upload a text file,and for eg the text file has some content for eg"This is a pen,this is a paper.That is a cat,that is a dog."User will be provided to enter any number,say for eg:3,means 3rd character of each line should be replaced with by it.So,in the first sentence its "This is a pen,this is a paper" and the 3rd character is "i".Hence, the whole sentence should be gone and should get back only "i".
PLease help me do this....