How do I change password within web.config

 
Hi,
I've stored my users (just two) in the web.config file, and I know I can get the current logged in user with User.Identity.Name, how can I change the password for the particular logged in user?
 
<authentication mode="Forms">
<forms loginUrl="login.aspx" name="ASPXAUTH" protection="All" timeout="30" defaultUrl="admin/index.aspx">
<credentials passwordFormat="MD5">
...