Forgot guest pw on cisco e2500 - Linksys By Cisco E2500 DualBand WirelessN Router

Do you have a computer which was used to configure your router initially? Using that computer , you can find out the guest network password.
You wil see cisco connect software on that computer.
Check this link for more information about Cisco Guest network access for Linksys

If a password was set by user, no one else will know it if you forget it.
If the password needs to be reset to factory, so you can log in with the "default" password AND RE-SETUP EVERYTHING, then you'll need to reset the Linksys E2500 to factory defaults. You can usually do that via a special button or sequence of actions.
The E2500 manual will have specific directions for performing a factory default reset. You can download the manual for the E2500 from http://support.linksys.com/en-us/support/routers/E2500