how to change the username and password in mysql

You need to use mysql (or mysql.exe on MS-Windows based system) under Linux or Unix like operating system. Open a terminal / ssh session. Type the following command at the shell prompt to login as a root user. The syntax is as follows for Unix like operating system:
$mysql -u root -h localhost -pOR use mysql.exe under MS-Windows host as follows (first change directory where mysql.exe is located [example: "C:\Program Files\mysql\mysql-5.0.77-win32\bin"] ):
mysql.exe -h localhost --user=root -pYou will see mysq> prompt as follows:
mysq>Syntax: Sql command to change a user password
Switch to mysql database (type command at mysql> prompt, do not include string "mysql>"):mysql> use mysql;The syntax is:

SET PASSWORD FOR 'user-name-here'@'hostname-name-here' = PASSWORD('new-password-here');

You can also use the following sql syntax:

UPDATE mysql.user SET Password=PASSWORD('new-password-here') WHERE User='user-name-here' AND Host='host-name-here';
 
In this example, change a password for a user called tom:
SET PASSWORD FOR 'tom'@'localhost' = PASSWORD('foobar');
OR
UPDATE mysql.user SET Password=PASSWORD('foobar') WHERE User='tom' AND Host='localhost';
Sample outputs:
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
Feel free to replace the values for "tom" (user), "localhost" (hostname), and "foobar" (password) as per your requirements. Finally, type the following command to reload privileges:
FLUSH PRIVILEGES;
Sample outputs:
Query OK, 0 rows affected (0.00 sec)
To exit from mysql> prompt, enter:
quit;
Sample session
Fig.01: Mysql Updating / Changing password (click to enlarge)