sql query difference between alter and update

Refer here,
http://sqlfortester.blogspot.in/2012/10/difference-between-update-and-alter.html[^]

If it's for upcoming interviews then, it's wrong way. You should learn SQL from start.
Go here http://www.w3schools.com/sql/[^]

ALTER - make changes in Table structure.
UPDATE - make changes in Table data.

ALTER STATEMENT
is part of Data Definition Language, is a syntax similar to a computer programming language for defining data structures, especially database schemas.

UPDATE STATEMENT
is part of Data Manipulation Language, is a family of syntax elements similar to a computer programming language used for inserting, deleting and updating data in a database.

UPDATE is DML used to modify existing data
ALTER IS DDL ,used to alter schema of objects like table

UPDATE Statement is used to update/change the 
data of row(s) only, we can use certain condition(s)using WHER clause
 UPDATE <Table-Name> SET <Column-Name(s)> WHERE <Column-Name> <Condition(s)>

Example: UPDATE Student SET DepartmentName='MCA' WHERE ClassId=3

ALTER: ALTER is used alter/change the SQL Table structure
1) Alter can be used to change the table column name, column type 
2) Alter also used to add new column or contraint to the table
3) It also used to delete (drop) column or contraint from the table