How can I change session timeout value?

Check the following values in your php settings.

1. session.cookie_lifetime
   - session.cookie_lifetime specifies the lifetime of the cookie in seconds which is sent to the browser.
   - a value of 0 means "until the browser is closed"
2. session.gc_maxlifetime
   - session.gc_maxlifetime specifies the number of seconds after which data will be seen as ’garbage’ and cleaned up.
   - garbage collection occurs during session start.

Try to set session.gc_maxlifetime to 7200. You must restart your server for this to take effect.