The following extra packages will be installed:
  mysql-server-5.5
Suggested packages:
  tinyca mailx
The following NEW packages will be installed:
  mysql-server-5.5
0 upgraded, 1 newly installed, 0 to remove and 58 not upgraded.
15 not fully installed or removed.
Need to get 0 B/1,981 kB of archives.
After this operation, 32.7 MB of additional disk space will be used.
Do you want to continue? [Y/n]
Preconfiguring packages ...
(Reading database ... 942067 files and directories currently installed.)
Preparing to unpack .../mysql-server-5.5_5.5.46-0ubuntu0.14.04.2_amd64.deb ...
Aborting downgrade from (at least) 10.0 to 5.5.
If are sure you want to downgrade to 5.5, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing archive /var/cache/apt/archives/mysql-server-5.5_5.5.46-0ubuntu0.14.04.2_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1
Errors were encountered while processing:
 /var/cache/apt/archives/mysql-server-5.5_5.5.46-0ubuntu0.14.04.2_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)