Replace a letter in a Textbox?

[C# Programming]

Hello.

I am creating a program, where i need to have the 29 letter in Textbox1 to be replaced by t

Also xxxxxxxxxxxxxxxxxxxxxxxxxxx(t)xxxxxxxxxxx

Also.. My users, shall paste a link, there is about 30-40 letters, and number 29 shall be changed to a t.

Is that possible?

Regards Ahlmo