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In order to change the settings of the Linksys router, you will need to login to Linksys router setup page.
If you have installed Linksys/Cisco connect software on your main computer then you can change settings using that installation program.
here is link which may help you with Linksys E2500 router.
https://fixhomenetwork.com/blog/linksys-e2500-router-setup/

The default SSID for linksys is Linksysxxxxx. "xxxxx" will be your model number. If you changed your SSID, then it will be whatever you changed it to.

Here's the link: http://bit.ly/K73ZqF

You could do a factory reset. This way, the unit will revert to the default username and password. You'll need to reconfigure all the settings though.

Check the quality of your internet, you can try continous ping command on a ip e.g:
open run: type cmd and hit enter
type: ping google.com -t

it should give reply....
check how many timed outs, that means droped packets,
could be internet issue,
If not check that you are not moving to far away from router range.

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

By default, the router has a SSID ID of 'linksys', with *NO* security, and *NO* password. Configure the router to enable security, and to set the SSID and to set a password.

Your router must be having port to connect it to computer - it may be either USB, or Ethernet port. use the cables respectively and connect it to your computer, on the same port which you use from router.

access the ipadd of your linksys then thats the time you can change your password. if you dont know the password then its time you reset your router and reconfigure it again.

Check whether u hav a securirty code to access internet..if u don have one then ur router can be accessed by others..