Visual Studio 2010 - Replace Variables Inside Txt File Programmatically From .net?

I need to read some information from a txt file and put all that info to a textbox (this is ready and done)but the textfile have some variables like %userName% that i need to replace programatically in vb.net?example of text file:hello %userName% , this is an automated report of the uses of your account. this is what the txt file have in it, when i read it to vb.net and put that info into a textbox is done right, but the question is how to replace the %userNams% for some text in another textbox and replaced when i press a button?