Modify web.config sessionState at run time

Hi, I set in my web.config a session timeout ...... <system.web> <sessionState timeout="20"></sessionState> ..... I wanna allow Administrator of website change the timeout for all users on the web.config by a Administrator's page, no manually, so I wanna know how to do this. I only can access informations of connections strings or appSetting on my code. thank's all ...