please i need favor !

example :

in file index.htm
<frame name="main" SRC="1.htm">
<frame name="content" SRC="2.htm">


code in 1.htm :
..
...
<input type="button" value="Press">

that i want to ask ?
when the button "Press" is pressed..i want
frame "Content" replace Src="2.htm" with Src="3.htm"

thanks alot