Preparing to unpack .../mysql-server-core-5.6_5.6.25-0ubuntu0.15.04.1_amd64.deb ...
Unpacking mysql-server-core-5.6 (5.6.25-0ubuntu0.15.04.1) ...
Selecting previously unselected package mysql-server-5.6.
Preparing to unpack .../mysql-server-5.6_5.6.25-0ubuntu0.15.04.1_amd64.deb ...
Aborting downgrade from (at least) 10.0 to 5.6.
If are sure you want to downgrade to 5.6, remove the file
/var/lib/mysql/debian-*.flag and try installing again.