$ sudo apt-get install mysql-server-5.5
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
  tinyca mailx
The following NEW packages will be installed:
  mysql-server-5.5
0 upgraded, 1 newly installed, 0 to remove and 19 not upgraded.
Need to get 0 B/1.744 kB of archives.
After this operation, 32,7 MB of additional disk space will be used.
Preconfiguring packages ...
(Reading database ... 630285 files and directories currently installed.)
Preparing to unpack .../mysql-server-5.5_5.5.41-0ubuntu0.14.04.1_amd64.deb ...
Aborting downgrade from (at least) 5.6 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_5.5.41-0ubuntu0.14.04.1_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_5.5.41-0ubuntu0.14.04.1_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)