Unpacking mysql-server-5.1 (from .../mysql-server-5.1_5.1.58-1ubuntu1_amd64.deb) ...
Aborting downgrade from (at least) 5.5 to 5.1.
If are sure you want to downgrade to 5.1, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing /var/cache/apt/archives/mysql-server-5.1_5.1.58-1ubuntu1_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1