GENERIC MYSQL INFO
To start with, read the mysql manual: https://dev.mysql.com/doc/refman/5.7/en/resetting-permissions.html
The steps will show you how to shut down the service and start it with an overriding command that doesn't require passwords, then you reset the password. From the manual:
Stop the MySQL server, then restart it with the --skip-grant-tables option. This enables anyone to connect without a password and with all privileges and disables account-management statements such as ALTER USER and SET PASSWORD. Because this is insecure, you might want to use --skip-grant-tables in conjunction with --skip-networking to prevent remote clients from connecting.
Connect to the MySQL server using the mysql client; no password is necessary because the server was started with --skip-grant-tables:
shell> mysql
In the mysql client, tell the server to reload the grant tables so that account-management statements work:
mysql> FLUSH PRIVILEGES;
Then change the 'root'@'localhost' account password. Replace the password with the password that you want to use. To change the password for a root account with a different hostname part, modify the instructions to use that hostname.
MySQL 5.7.6 and later:
mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY 'MyNewPass';
MySQL 5.7.5 and earlier:
mysql> SET PASSWORD FOR 'root'@'localhost' = PASSWORD('MyNewPass');
Or directly on the user table:
UPDATE mysql.user SET password=PASSWORD('mynewpassword') WHERE user='root';
XAMPP SPECIFIC
Stop the MySQL service. Open a command window. Change to the XAMPP MySQL directory:
> cd \xampp\mysql\bin\
Run the service without security (note you are running mysqld, not mysql):
> mysqld.exe --skip-grant-tables
The MySQL service will be running in this window, so open another command window and switch to the XAMPP MySQL directory:
> cd \xampp\mysql\bin\
Run the MySQL client:
> mysql
Update the password:
mysql> UPDATE mysql.user SET password=PASSWORD('mynewpassword') WHERE user='root';
Exit MySQL:
mysql> \q
Use task manager to cancel the mysqld.exe that is still running. Restart the mysql service.