If it's a server control on a web page, then you can't do it. The server controls only exist while the page is being created, and only one page at a time is created for each user. The controls never exist at the same time. If it's an element on a web page, you have to use Javascript. If it a control on a form in a program, you need a reference to the form where the control is that you want to change.
--- single minded; short sighted; long gone;