Upgrade MAMP 4 to Mysql 5.7.18 (on Sierra tested)

migrate.sh

#!/bin/sh

wget http://dev.mysql.com/get/Downloads/MySQL-5.7/mysql-5.7.18-macos10.12-x86_64.tar.gz
tar xfvz mysql-5.7.18-macos10.12-x86_64.tar.gz

echo "Stopping MAMP"
sudo /Applications/MAMP/bin/stop.sh
sudo killall httpd mysqld

echo "Copy Bin"
sudo rsync -arv --progress mysql-5.7.*/bin/* /Applications/MAMP/Library/bin/ --exclude=mysqld_multi --exclude=mysqld_safe 

echo "Copy Share"
sudo rsync -arv --progress mysql-5.7.*/share/* /Applications/MAMP/Library/share/

echo "Building Mysql 5.7 Folder"
sudo cp -r /Applications/MAMP/db/mysql56 /Applications/MAMP/db/mysql57
sudo rm -fr /Applications/MAMP/db/mysql57/mysql/innodb_*
sudo rm -fr /Applications/MAMP/db/mysql57/mysql/slave_*
sudo chmod -R o+rw  /Applications/MAMP/db/mysql57/
sed -i.bak 's/mysql56/mysql57/g' /Applications/MAMP/Library/bin/mysqld_safe

echo "Fixing Access (workaround)"
sudo chmod -R o+rw  /Applications/MAMP/tmp/mysql/

echo "Starting MySQL"
sudo /Applications/MAMP/Library/bin/mysqld_safe --port=3306 --socket=/Applications/MAMP/tmp/mysql/mysql.sock --pid-file=/Applications/MAMP/tmp/mysql/mysql.pid --log-error=/Applications/MAMP/logs/mysql_error_log

echo "Migrate, finaly, to new version"
sudo /Applications/MAMP/Library/bin/mysql_upgrade --user=root --password=root --port=3306 --socket=/Applications/MAMP/tmp/mysql/mysql.sock --force

@julian-tr 's solution above worked for me

My solution was to simply duplicate the mysql56 folder in the Finder, rename the copy to mysql57 and start MAMP. It then offers to upgrade the databases for you.

If it helps I used the following:

#!/bin/sh

echo "Getting MYsql 5.7"
wget https://dev.mysql.com/get/Downloads/MySQL-5.7/mysql-5.7.23-macos10.13-x86_64.tar.gz
tar xfvz mysql-5.7.23-macos10.13-x86_64.tar.gz

echo "Stopping MAMP"
sudo /Applications/MAMP/bin/stop.sh
sudo killall httpd mysqld

echo "Copy Bin"
sudo rsync -arv --progress mysql-5.7.*/bin/* /Applications/MAMP/Library/bin/ --exclude=mysqld_multi --exclude=mysqld_safe

echo "Copy Share"
sudo rsync -arv --progress mysql-5.7.*/share/* /Applications/MAMP/Library/share/

echo "Building Mysql 5.7 Folder"
sudo cp -r /Applications/MAMP/db/mysql56 /Applications/MAMP/db/mysql57
sudo rm -fr /Applications/MAMP/db/mysql57/mysql/innodb_*
sudo rm -fr /Applications/MAMP/db/mysql57/mysql/slave_*
sudo chmod -R 0777  /Applications/MAMP/db/mysql57/
sed -i.bak 's/mysql56/mysql57/g' /Applications/MAMP/Library/bin/mysqld_safe

echo "Fixing Access (workaround)"
sudo chmod -R o+rw  /Applications/MAMP/tmp/mysql/

echo "Updating MAMP socket"
sudo /Applications/MAMP/Library/bin/mysql_config_editor --verbose set --socket=/Applications/MAMP/tmp/mysql/mysql.sock

echo "Starting MySQL"
sudo /Applications/MAMP/Library/bin/mysqld_safe --port=3306 --socket=/Applications/MAMP/tmp/mysql/mysql.sock --pid-file=/Applications/MAMP/tmp/mysql/mysql.pid --log-error=/Applications/MAMP/logs/mysql_error_log

echo "Migrate, finaly, to new version"
sudo /Applications/MAMP/Library/bin/mysql_upgrade --user=root --password=local --port=3306 --socket=/Applications/MAMP/tmp/mysql/mysql.sock --force

@ronappleton 's code works fine for me. Thanks man!