If you want update Mysql from 5.6 to 5.7 do this steps
https://dev.mysql.com/downloads/repo/apt/
Download mysql-apt-config_0.6.0-1_all.deb
Install apt-sources
dpkg -i mysql-apt-config_0.6.0-1_all.deb
After that you have apt-resources for installing
Update resources
apt-get update
Install Mysql server
apt-get install mysql-server
And upgrade mysql tables
mysql_upgrade
Also you can use another method with repository ondrej
$ sudo apt-get install software-properties-common
$ sudo add-apt-repository -y ppa:ondrej/mysql-5.7
$ sudo apt-get update
$ sudo apt-get install mysql-server
Also read wthat new in 5.7
https://dev.mysql.com/doc/refman/5.7/en/upgrading-from-previous-series.html