When you installed MySQL for the first time, you can't login with a no-password.
You should add the following line in my.cnf to login without a password.

skip-grant-tables

It has a huge security risk, so that you should set a password to prevent external, illegal access.

How to Retake Control of the Root User in MySQL

Step1: Connect to MySQL

mysql

Step2: Using the main database

use mysql;

Step3: Redefine user root password

UPDATE user SET `authentication_string` = PASSWORD('myNuevoPassword') WHERE `User` = 'root';

Caution
If you can't execute the command, you should check authentication_string column of mysql.user table.

SELECT User, authentication_string  FROM mysql.user\G
*************************** 1. row ***************************
                 User: mysql.infoschema
authentication_string: $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED
*************************** 2. row ***************************
                 User: mysql.session
authentication_string: $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED
*************************** 3. row ***************************
                 User: mysql.sys
authentication_string: $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED
*************************** 4. row ***************************
                 User: root
W&2-*D:1FsLlMJEFsvaZNpaAlnJDEyTNo2QO7Eu7P0rhe10psBsC
4 rows in set (0.00 sec)

And you execute the following command.

UPDATE user SET `authentication_string` = 'myNuevoPassword' WHERE `User` = 'root';

Step4: Show mysql.user table if you could change root password

SELECT User, authentication_string  FROM mysql.user\G
*************************** 1. row ***************************
                 User: mysql.infoschema
authentication_string: $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED
*************************** 2. row ***************************
                 User: mysql.session
authentication_string: $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED
*************************** 3. row ***************************
                 User: mysql.sys
authentication_string: $A$005$THISISACOMBINATIONOFINVALIDSALTANDPASSWORDTHATMUSTNEVERBRBEUSED
*************************** 4. row ***************************
                 User: root
authentication_string: ZAQ$2wsx2408
4 rows in set (0.00 sec)

Step5: Erase line from my.cnf

skip-grant-tables

Step6: Restart MySQL service

sudo systemctl restart mysqld

Step7: Connect to mysql with password

mysql -u root -p
Enter password: [new password]

mysql> 

When you need to do this action, please use carefully!