A router usually has internal flash to store its firmware and limited memory. Thus the webpages you see are almost guaranteed to be stored in the flash.
To change them you need to:
- Get a copy of the current firmware.
- Unpack it so that you can edit it.
- Edit it
- Repack it as firmware.
- Flash the modem with the new firmware and hope you did not make any errors. If you made an error you might have just bricked your Beetel.
So, the practical answer is: No, you can not just edit them.
You can look if there is a firmware available for your device which allows you do make easier changes (See a list of those here). Howevereven with a relative open firmware changing the webpage layout is non-trivial.