Syntax:
ALTER USER [IF EXISTS]
    user [auth_option] [, user [auth_option]] ...
    [REQUIRE {NONE | tls_option [[AND] tls_option] ...}]
    [WITH resource_option [resource_option] ...]
    [password_option | lock_option] ...

ALTER USER [IF EXISTS]
    USER() IDENTIFIED BY 'auth_string'

user:
    (see )

auth_option: {
    IDENTIFIED BY 'auth_string'
  | IDENTIFIED WITH auth_plugin
  | IDENTIFIED WITH auth_plugin BY 'auth_string'
  | IDENTIFIED WITH auth_plugin AS 'auth_string'
}

tls_option: {
   SSL
 | X509
 | CIPHER 'cipher'
 | ISSUER 'issuer'
 | SUBJECT 'subject'
}

resource_option: {
    MAX_QUERIES_PER_HOUR count
  | MAX_UPDATES_PER_HOUR count
  | MAX_CONNECTIONS_PER_HOUR count
  | MAX_USER_CONNECTIONS count
}

password_option: {
    PASSWORD EXPIRE
  | PASSWORD EXPIRE DEFAULT
  | PASSWORD EXPIRE NEVER
  | PASSWORD EXPIRE INTERVAL N DAY
}

lock_option: {
    ACCOUNT LOCK
  | ACCOUNT UNLOCK
}

The ALTER USER statement modifies MySQL accounts. It enables
authentication, SSL/TLS, resource-limit, and password-management
properties to be modified for existing accounts. It can also be used to
lock and unlock accounts.

To use ALTER USER, you must have the global CREATE USER privilege or
the UPDATE privilege for the mysql system database. When the read_only
system variable is enabled, ALTER USER additionally requires the SUPER
privilege.

By default, an error occurs if you try to modify a user that does not
exist. If the IF EXISTS clause is given, the statement produces a
warning for each named user that does not exist, rather than an error.

URL: https://dev.mysql.com/doc/refman/5.7/en/alter-user.html

