You can upgrade by using below commands
#apt-get upgrade mysql*
But MySql 5.5 is the default for Ubuntu 14.04. For upgrading MySQL you should configure MySQL repository, if you want to update MySQL-5.5 to for ex MySQL-5.7 you should upgrade to MySQL 5.6 first before upgrading to 5.7.
You can download the repositories from below link
and for installing repositories follow the below article
after upgrading mysql-5.5.x to 5.6.x using mysql repository 5.6.x, you should disable 5.6.x repository and enable 5.7.x repository for upgrading 5.6.x to 5.7.x
for disabling repo
by checking and unchicking the boxes you can disable or enable the repo's
If you have data in your MySql 5.5 database, it should be migrated automatically, even it's better to make a backup before doing a major upgrade.
#mysqldump --lock-all-tables -u root -p --all-databases > dump.sql
for restoring the data
#mysql -u root -p < dump.sql