The <sql:update> tag in JSTL (JavaServer Pages Standard Tag Library) is used to execute an SQL UPDATE statement. Here's an example of how to use it:

<%@ taglib uri="http://java.sun.com/jsp/jstl/sql" prefix="sql" %>

<sql:setDataSource
  var="myDataSource"
  driver="com.mysql.jdbc.Driver"
  url="jdbc:mysql://localhost:3306/mydatabase"
  user="myuser"
  password="mypassword"/>

<sql:update dataSource="${myDataSource}">
  UPDATE mytable SET column1 = 'newvalue' WHERE column2 = 'value2'
</sql:update>

In this example, we first define a data source using the <sql:setDataSource> tag. We then use the <sql:update> tag to execute an UPDATE statement on the mytable table, setting the value of column1 to 'newvalue' where column2 equals 'value2'.

The <sql:update> tag can also be used to execute DELETE and INSERT statements, as well as other types of SQL statements. The result of the SQL operation is not returned by the <sql:update> tag, but can be checked using the getUpdateCount() method of the JDBC Statement object, which is made available by the JSTL SQL tag library.

Note that JSTL SQL tags have been deprecated since JSTL 1.2, so it is recommended to use a different method for database access, such as JDBC or an ORM (Object-Relational Mapping) framework like Hibernate.