Is there any way to downgrade 10.10 to 10.04 without reinstalling linux ?
It's possible at the level of the packaging tools (apt). But the resulting system may not be equivalent to doing a reinstall, and you may get errors along the way. This is because many packages contain specific support for upgrades (e.g. handling changes in configuration files) but not for downgrades.
Normally, apt prefers to install the most recent version of a package. But you can change this through pinning: you can declare that packages from the old release have higher priority than the installed packages, so that they will be downgrades when you do